Hey all!
Perhaps you are new to Halo as a franchise, new to video games, or a returning fan who just hasn't played in a while, this guide is a detailed description of the various difficulty levels in the series, mixed with some tips and tricks that will help you along the way.

The amount of options can be intimidating to someone inexperienced with the franchise as the difficulty in-game descriptions are, at best, vague (they're funny enough though). But I hope to use my experience to help clear up what choice someone may want to make.

A thing to note is, regardless of the game the following modifiers (noted as damage modifiers throughout this guide) are affected the higher the chosen difficulty is: Enemies do more damage, enemies fire rate is increased/have less pauses between firing, enemies have more health/spawn in more numbers, enemies with higher ranks become more common, enemies awareness or the player is increased, enemies reaction times are decreased.

A side note, however is that I will not be covering EASO, NASO, HASO or LASO* (advanced custom difficulty settings) in this guide. As not all skulls are present in all games and that would be a whole other guide on its own, in my opinion, to explain what modifiers they affect in-game. Albeit they exist in 2 categories (scoring and non-scoring) and affect several game modfiers depending on if they are turned on or off. I want to remind you all that no matter your skill level, Halo is a franchise that is welcoming to all players. Don't feel bad if you struggle at higher difficulties, as these options exist for a reason.

Without further ado, let's get into it.

*EASO: Easy All Skulls On; NASO: Normal All Skulls On; HASO: Heroic All Skulls On; LASO: Legendary All Skulls On.

Halo Combat Evolved is the first game in the franchise and is, by far, the best balanced and therefore has the most "fair" difficulty of any of the games. For this reason, my recommendations may vary from future games. I will be listing bullet points that indicate reasons you may wish to choose a difficulty in question, as well as a brief overview of what that difficulty is like.

Some things to keep in mind about Halo CE on all difficulties:
-The Shotgun is the strongest weapon in this game, you always want to grab one "for close encouners", specially on the later acts of the game where it becomes esponentially more common (It loads 12 shells plus 60 in reserve and has by far the most overpowered Shotgun stats in the franchise, allowing for 1 to 2 shot kills at ranges no other future Shotgun in Halo could ever rival). The Shotgun is also extremely reliable against both flesh and shielded enemies. This weapon is so good that elevates the use of secondary weapons you would normally dismiss, especially on lower difficulties. That is how good the weapon is, it is a weapon that does not need to rely so closely on others to get the job done.
-The God Gun (aka Magnum) is the jack of all trades weapon in this game. It excels at most combat scenarios but it is not strong enough at any particular scenario as to make the other guns obsolete. The Shotgun is stil the best at close range, AR is the best trash cleaner and melee weapon in the game, The Plasma Pistol is the best for depleting shields on shielded enemies and so on.
-The Plasma PIstol is the best weapon to deal with shields. It melts shields with regular fire, can insta drain shields with a charged shot, has five HUNDRED trigger pulls from a full charge, has impressive range, and can fire as fast as you pull the trigger. There are only 3 downsides of the CE plasma pistol and that is that the charged shots tracking capabilities are sometimes unreliable/random, the dependency on a precision weapon to deal the final killing blow against enemies with headshot capabilities and the weapon's poor match-up against Hunters. Overall though this downsides are nothing compared to its many upsides.
-If a Plasma Pistol is not immediately available a good alternative to strip shields is the Plasma Rifle. This is a semi-automatic Covenant weapon carried by all Elite types (excluding Zealots with Energy Swords). However, the weapon falls considerably the higher the difficulty is as it gets overshadowed by the more common and reliabely better Plasma Pistol. There are, however, instances in the game where the Plasma Pistol will be unavailable. It comes to my mind the last part of mission 8, where you can rarely get a Plasma Pistol up to that stage and you will face shielded enemies and the beginning of the last mission where you spawn with an AR and Plasma Rifle. So again the Plasma Rifle is a very situational weapon.
-Don't pick up a Needler. Just don't. The Needler is very viable in many later entries but the projectile speed and tracking is so slow in CE that if you're playing on any difficulty higher than Normal enemies will easily melt you before you get viable kills with it.
-The AR is a good trash cleaner and can out-damage many other options at close range. That combined with its high maximum ammo count makes it a good workhorse.
-The orange part of a Hunter counts as their head and for that reason two headshot capable weapons in the game (The Sniper Rifle and Magnum) one shot kill the Hunters in these areas. This will absolutely trivialize them.
-A Sniper Rifle is useless in later acts of the game against most of the enemy cast. Carrying one will be very situational and the weapon falls off dramatically here. This is the weapon that falls off the most after you finish act 1 of the game. A niche unique to CE is the weapon has a hidden stealth meter. Meaning it may be loud but if fired onto a crowd of enemies that are not alerted to your presence and they will still keep on just patrolling the area or panic from fallen enemies before resuming patrol stances.
-The Rocket Launcher is also a very situational weapon. You do not want to carry one most of the time as you are just saving rockets for specific fights and its only really a necessity in the last level and the 8th mission first half. Albeit the weapon is still a good portable grenade launcher but has a unique trait only seen is CE which is fire delay between rockets. Beware of this when using the weapon.
-Halo CE is the only game to feature motion tracker support while zooming with a scoped weapon. Use this knowledge to your advantage. The exception is Halo 4 multiplayer loadout system that has the "Awareness" perk that does the same thing as Halo CE does.
-Unique to Halo CE are melee damage and range properties. Every weapon has different damage and range properties (damage and range are unique to every single weapon in CE, lining up with the animations), meaning the AR has different properties than a Magnum, for example. It is not just a flashy animation. AR has the best melee speed in the game (and deals good amounts of damage per strike) followed by the Rocket Launcher. On the worse side we have the Magnum (its the slowest and does pitiful melee damage per strike). It is the worst melee option in the game, which is a shame because the animation is damn good.
-Hunters in CE can be hurt by melee. Meaning you can bash them to death with the AR while you fire the weapon at the same time. It will get the job done if you get bored of just 1 shot killing their "headshot region" and will be somethng worth trying, especially on lower dificulties and/or on a relaxed run.
-Every single enemy has different health values depending on both difiiculty modifiers and base health per rank. An example is Red Elites have approximadely 50% stronger shields than Blue Elites. This is a trait unique to only 2 games, as far as I am aware, which are Halo CE and Halo Reach. Another example is Orange Grunts base health being weaker than Red Grunts base health plus the difficulty modifiers.
-Always be mindful of where your Marine allies are at all times. They are an absolute menace in Halo CE with frag grenades or when piloting Ghosts. You might die to a random frag grenade thrown by a Marine that wipes his whole squad and you or be run over by a Marine that decided it was a good day to test drive a Ghost. I can not tell you how many times I got killed by a non-hostile Marine either lobbing a random frag grenade that caused a chain reaction or a Marine pilotted Ghost. Its funny and annoying at the same time.

Halo Combat Evolved's higher difficulties will seem intimidating to first time players but I'll include some advice to help you through it.
If you're new to Halo or new to shooters, Halo CE's higher difficulties are definitely the first step to getting better at Halo as they are inherently more balanced than some of the later entries.

Easy:
Easy's name says it all. In this mode, enemies have very little health, and do far less damage to the player. This allows for an experience that is not punishing even for those without technical skill at video games. There is no shame in playing on Easy. Easy is a good choice for people who are new to video games in general and thus may need a mode that is very forgiving. It allows accessibility to the game for those who otherwise might not be able to play it.

The main reasons to play on Easy are:
-Speedrunning for the Par Time achievements;
-New to video games as a whole;
-Learning the controls/mechanics;
-Only care about the story and not the gameplay.

Normal
Normal is the next difficulty up from Easy and is set as the "standard" by 343i. Bungie considered Heroic to be the way the games are meant to be played but 343i seems to have chosen to balance around Normal, so for this reason you will find playlists for the games are often set to this difficulty.
Normal is not an overly difficult experience and will become very easy once someone is familiar with the game's mechanics and controls. Despite this, there are perfectly valid reasons to play on Normal no matter what the skill level of the player is. Damage to the player is at it's default value and enemy health is reasonable. Normal is more forgiving of mistakes than higher difficulties but you will find yourself dying if you make mistakes that Easy would allow.

The main reasons you may choose normal are:
-Playing the playlists built into MCC;
-Trying to get through levels at a reasonable pace;
-You hate the Rocket Flood as much as the rest of the community does;
-Being a first time player of shooter games but not of video games as a whole. If you're already comfortable with a keyboard/mouse or controller but don't have experience with the shooter genre, Normal may be a good starting point;
-Wanting to learn strategies for approaching enemies. Normal is a good tool for learning enemy weaknesses.

Heroic
As Heroic's description in-game says, Heroic is the way Halo is meant to be played. It is a decently challenging experience that will still forgive minor mistakes.
Enemies have slightly boosted health and do a bit more damage. Some weapon combinations that were feasible on Normal or Easy are not as valuable on Heroic. The most obvious example being the Needler which will be practically useless within a larger crowd of enemies due to the Halo CE Needler being the slowest and arguably weakest in the series.
On Heroic, enemies of higher ranks become somewhat more common and you will at times face more enemies than you would on Normal or Easy.
Many of the achievements for different in-game feats require you to play on at least this difficulty.

Reasons to play on Heroic may include:
-Bungie's intent was for this to be the default experience and thus for the sake of artistic intent you may wish to play this.
-You love the Rocket Flood annoying the hell out of you but can tolerate a moderate amount of BS;
-This is a good choice for first time Halo players who have experience with first person shooters as a genre as it provides a fine amount of difficulty and will be about on par with what you'd expect for what reflexes are expected of you.

Legendary:
Legendary is the highest difficulty included by default.
Legendary is the least forgiving difficulty. Enemies do much higher damage, have way higher health, and there are more of them and higher ranks are the most common. You will find that marines are even more useless than ever before as they will struggle even against grunts.
However, since Halo CE is well balanced Legendary is firm but fair and you will not find yourself dying to unreasonable things very often. Almost all mistakes can be avoided by correcting your approach to scenarios.
My biggest point of advice to people playing Halo CE on Legendary is two important pieces of information: Do not underestimate any enemy as they can overwhelm you very quickly if you make mistakes and always carry a Plasma Pistol with you to deal with every shield you might come across.
Legendary is a rite of passage in the community, so to speak.

Reasons to play Legendary are:
-Legendary-only achievements;
-Good for those seeking a challenge;
-Heroic no longer provides adequate challenge for your skill level in terms of reflexes;
-You have mastered the mechanics;
-You love the Rocket Flood annoying the hell out of you;
-You want to experience what the war probably felt like from the first-person prespective.

So, you've made it through CE and are onto Halo 2. Halo 2's development was quite a mess, and there are many documentaries on it should you be interested. But for the sake of this guide the only thing you need to know is that Halo 2's balance is....poor, at times.
Halo 2 is well beloved in the community for its amazing story telling and ambitious feature list, but you may find yourself struggling in ways you didn't before.
For that reason, you may find yourself enjoying Halo 2 on a lower difficulty than you did Halo CE. The specific reasons for which will be delved into in each section.
As a general rule about entries from here on, Halo 2 is when the franchise began to lean into the "precision meta" way more, meaning that you will find that headshot capable weapons dominate the franchise going forward, to the point it will often feel pointless to bring anything else.

Some things to keep in mind about Halo 2 on all difficulties:
-Vehicle combat is broken in Halo 2. When in a vehicle or against a vehicle, your character shields mind as well be made of vegetable paper and your vehicle hull seems to be more rapidly decaying the harder the difficulty is. Be very mindful of that in vehicle sections, especially on higher difficulties.
-Don't bring the SMG. It is worthless. It might as well be chambered in .22 birdshot for how much damage it'll pull at any difficulty higher than Easy or Normal.
-The Needler is good now. Really good. It's one of the only viable non-precision weapons on higher difficulties. It has better tracking, faster firing and supercombines melt enemies. It is also the best dual-wielding weapon by far. Spread that pink mist around from 2 Needlers. The only downside is that when dual-wielded you cant melee, throw grenades or board vehicles without dropping your weapon in the other hand.
-They turned the glorious Halo CE magnum, the M6D, into the M6C or as I like to call it the Supersoaker someone forgot to make orange. Don't touch it. It might as well be chambered in .18 paintball for how much damage it'll pull at any difficulty.
-The Plasma Pistol got nerfed. It's still definitely worth bringing for shields but don't try to use it to score kills on its own anymore. However, the trade-off is the tracking was way buffed so it is only viable to do one thing and one thing only: drain shields with the overcharged shot.
-Elites got a huge buff in this entry. Close quarters is not ever a good idea with Halo 2 elites, as they will insta kill you on Normal difficulty and above (although their melee is still dogeable). Unless it's Heretic with a Sentinel Beam, because they do zero Melee damage due to a glitch.
-Brutes are essentially bullet spunges and boxing champions. Bringing anything that is not a Carbine or Battle Rifle will result in death on difficulties higher than Easy. Close quarters with a Brute is again a pain.
-Halo 2 and 3 are the only 2 games in the MCC where the Elites speak in English. They have memorable quotes and make them far more expressive than in Halo CE, especially as allies.
-Halo 2 is when the franchise started to loose lunge properties. Now all weapons do the same lunge damage regardless of anmation. Exception are the Enegy Sword main attack which is by far the best anti-Flood weapon in Halo 2. However, the game has a unique mechanic where there is a certain degree of "lunge assist". This is most noticeably felt while using a weapon that is not an Energy Sword. You will feel a certain "weight" when doing melee combat, like if the game had an hidden target marker on the enemy and your character just locks on it when performing a melee move.
-Halo 2 also lost the unique traits each enemy had per rank. Now the following ranks all have the same health and shield values (Elite Minor, Elite Ranger, Stealth Elite have the weakest shields and health values, followed by Elite Majors and Honor Guard Elites. Elite Ultras have the highest values of health and shields, to a point they possess overshields in this game). A similar rule applies to the remaining groups you fight in the Campaign.
-Hunters once again are glitched in Halo 2. This time you can stun lock them with a plasma grenade stick, forcing them to expose their weak point for a few seconds. They can no longer be hurt by melee or 1 shot killed by precision weapons but power weapons like a Sniper Rifle will take a few shots to put them down.
-Halo 2 introduced weapon swapping with your allies. They will have infinite ammo with any weapon you give them and with some weapons they will even have hidden exclusive perks (like auto-lock on to vehicles with a Fuel Rod while the player can not do this). So use this mechanic to your advantage. They will spam the hell out of certain weapons making combat easier for you, even on higher difficulties. Marines especially with Rocket Launchers also have a unique trait where they will refuse to fire the weapon if they are within suicidal range of an enemy. Be very mindful of them when giving them this specific weapon for close-quarters combat as they will die in certain levels simply because of this trait. Even if they attempt to get further away from an enemy they will still get overwhelmed, even on lower difficulties.
-Halo 2 also introduced the ability for allies to pilot multi-manned vehicles, allowing the player to board as either Gunner or Passenger. However, this mechanic is serviceable and useless on any difficulty above Normal as the AI struggles to navigate the map and know when to charge or retreat from a fight. Its not perfect but its a welcome addition that would continue to service Halo games on the MCC going forward.
-Halo 2 is the only game in the MCC to have multiple Sentinel Ranks (Minor Aggressor Silver, Major Agressor Gold, Sentinel Enforcer that can crush vehicles) as well as some unique Elite Ranks as allies such as the Elite Councillors (have overshields and don’t falter during melee combat) and stealth Elite Minors and Majors and Honor Guard as enemy Elites. It’s the only game where you fight alongside Jackal and Hunter allies. There are 2 variations of the Sentinel Beam as well. The regular from Minor Silver that shoots a yellow beam and the Major Gold Sentinel Beam which fires a stronger blue beam (Halo 3 would only feature the regular Sentinel Beam that shoots a yellow beam). It’s also the only game where both versions of the Plasma Rifle exist (Normal Blue that shoots blue plasma) and Brute Red (that shoots red plasma). The only difference between them is how fast they overheat (the Brute variant overheats faster). Both these weapons will continue to appear in the games but not together: Normal Blue (Halo CE, 2, 3, Reach), Brute Red (Halo 2 and ODST). Funny enough the Brute version is introduced in the worst way possible as Brutes are basically bullet spunges involnerable to non-headshot capable plasma weapons, the higher the difficulty is so making the weapon nearly useless against them. Just don’t pick one if you see it. Even with dual-wielding the weapon will underperform simply because Brutes don’t have shields but huge amounts of flesh that make them bullet spunges invulnerable to plasma, except for the headshot capable plasma weapons.
-From Halo 2 onwards different weapon melee damage and range are replaced with the lunge system. Specific weapons will have buffed melee damage (like the Energy Sword or Brute Shot) but for the most part the lunge values will be the same per melee strike.

Easy:
Easy remains more or less unchanged from Halo CE. And therefore my recommendations for why you'd play it are more or less unchanged. This mode is still good for beginners to video games as a whole and for those looking to speedrun.
Easy will not be very fulfilling gameplay wise as it practically plays itself in Halo 2, especially since your allies AI is somewhat improved compared to the previous entry.

You may choose Easy in Halo 2 if:
-You are going for Par Time achievements;
-You are new to video games as a whole;
-Only care about the story and not the gameplay.

Normal:
This is where the guide changes from the Halo CE section. Halo 2's Normal difficulty is only slightly easier than Halo CE's Heroic difficulty. For example, elite melee is an instant kill even on Normal and you will likely find yourself still dying on occasion, moreso than on Normal in Halo CE. For that reason, my list of reasons to play on Normal will be slightly different than for CE.
Generally the principles of it remain the same. It simply requires more reflexes than Easy. The primary difference between Halo CE's normal and Halo 2's Normal is that there's more things that can instantly kill you even on Normal in Halo 2.

You may play Halo 2 on Normal if:
-You are displeased with the imbalances of Halo 2 but still want to enjoy it's story and gameplay;
-Playing the playlists built into MCC;
-You are new to Halo as a Franchise but have experience with video games;
-You hate Jackal Snipers as much as the rest of the community does;
-You hate the Rocket Flood as much as the rest of the community does;
-Brutes are as annoying to you in Halo 2 as they are to the rest of us.

Heroic:
Heroic in Halo 2 is where things start to get a bit..different. Enemies do far more damage and at times can melt you in seconds, making this difficulty far less forgiving of mistakes than Heroic on Halo CE.
Generally I would recommend you are very familiar with the weapons sandbox before attempting this as Halo 2's enemies are far more polarized to their weaknesses and strengths than in Halo CE. Facing a brute without a headshot capable weapon can be daunting at the best of times.
You will find yourself dying on occasion to things that seem entirely contrived. Jackal snipers are far more common and have many more spawns on Heroic than on Normal. Brutes are bullet sponges unless you bring a precision weapon, and elite melee is, as you guessed, still instant kill.

You may play Heroic on Halo 2 if:
-You enjoy the core gameplay of Halo 2 despite it's quirks and can forgive occasional BS;
-You want achievements locked to Heroic or higher;
-You are comfortable with the weapon sandbox and have enough knowledge of enemy weaknesses to stack up;
-You want to respect the artistic intent and play Halo 2 "as intended";
-You find Normal not challenging enough but don't hate yourself enough for Halo 2 Legendary.

Legendary:
This is the section most of you were waiting for. Halo 2 Legendary is best described as a joke at the expense of the player. Jackal Snipers are almost always instant kill and will snap shot you in under a second if you are not careful. Brutes are essentially invulnerable except for headshots. Elite shields feel like they're powered by nuclear generators, allowing them to survive direct rocket launcher shots. Ultras and Zealots are everywhere (Zealots only spawn on this difficulty). Drones are the spawn of Satan and the Rocket Flood can lock onto your vehicles. This time around you’re the second weakest entity in the game, just stronger than flood infection forms that die in a single hit so the only viable strategy is pick off enemies one by one from behind cover, memorize Jackal Sniper and every single other enemy spawns so you don’t get caught off-guard and melted in seconds by anything that isn’t a flood infection form and also avoid close quarters combat entirely. Because of the damage modifiers on top of player character handicaps the TTK (Time To Kill: the time it takes to kill a character) is the lowest of all the MCC games on Legendary.
Your allies might as well be your little brother without arms holding the controller because they will be melted into puddles of blood and regret before you can even get to a first name basis with them.
If you must play on Legendary, the Noob combo remains reliable. Take the Battle Rifle/Carbine and Plasma Pistol, drain Elite shields with the Plasma Pistol and then go for a quick headshot. The tracking on the plasma pistol is somewhat more reliable than in Halo CE, though in all other ways the plasma pistol is downgraded. It isn't the workhorse it was in CE, so only rely on it for shields.

You may play Halo 2 on Legendary if:
-You wish to feel vulnerable;
-You love Jackal Snipers annoying the hell out of you;
-You love the boxing heavy weight champion Brutes;
-You love the spawn of Satan aka Drones;
-You love the Rocket Flood newfound powers of annoyance;
-You want the Legendary achievements that definitely aren't worth the suffering;
-Actual war wasn't challenging enough for you;
-You are thrill seeking.

Halo 3 returns to normal after Halo 2. The difficulty imbalance is addressed and overall the game is incredibly polished. I will say it's not AS balanced as CE but it's still quite fair. Three is a fan favorite gameplay-wise for a large section of the community.
Difficulty wise it's easier than Halo 2 but has less balanced than CE.
Generally it continues the trend of a precision meta, though Halo 3 also places much more emphasis on the use of power weapons and vehicle disabling equipment.

Some things to keep in mind about Halo 3 on all difficulties:
-Vehicle combat is broken in Halo 3. When in a vehicle or against a vehicle, your character shields mind as well be made of vegetable paper. Grunt piloting Ghosts are a nightmare so they are incredibly annoying and frustrating thanks to the new ability of removing the Grunts methane tanks with weapons. Its a good idea to carry a Plasma Pistol, EMP those pesky Ghosts then go for the kill that way with either Plasma or Spike Grenades, Equipment like a Power Drain or a well-placed Trip Mine. It's also a good idea to carry a Rocket Launcher, Sniper, or Spartan Laser if you can find one.
-Trip Mines will Instantly detonate if a direct hit happens when you throw one. Use this to your advantage to insta kill any small scale vehicles like Choppers or Ghosts. Otherwise they will just stay there active and can hit friend or foe.
-The Power Drain can kill the vehicle pilot and immobilize the vehicle for its duration. This is absolutely helpful if you dont want to waste Plasma Pistol Energy to EMP a vehicle.
-Starting in Halo 3 and going forward in MCC the Plasma Pistol can now EMP all eletronics (shields and vehicles alike) with the overcharged shield. However, its useless against power armor because that armor will not regenerate once its broken (better weapons to strip power armor faster like the AR exist in the game, or any meta precision weapon like the BR and Carbine). Nevertheless it remains as good as ever, even if it was downgraded in the rest of the stats. It also drains the battery while an overcharge shot is held by the player, which will keep happening in the remaining MCC entries going forward (making it more of a strategic weapon than it was in the first 2 Halo Games). You have to learn when to overcharge with this Plasma Pistol otherwise you will run out of battery faster than you can say "Finish the Fight".
-The Flare is both bugged (in MCC) and useless. Its blindiness is unusually high in MCC to the point of causing migraine inducing moments when the enemies use it against you and the thing is useless in your hands because you might just wind up blinding yourself as you are the enemies. There is no reason to ever carry a Flare. Ever.
-The Spiker is possibly the contender for worst weapon in Halo 3 (along with the Magnum). It fires shards that mind as well be made of red paper. It does pitiful amounts of damage even on lower difficulties. Unless you are experimenting do not use it. It is an awful weapon, and dual-wielding will not fix this.
-The Needler is God tier now. It can no longer be dual-wielded but is by far the best Needler in the franchise (Along with Halo 3 ODST Needler). The Needles travel extremely fast and can melt anything in seconds, except Hunters.
-Halo 2 and 3 are the only 2 games in the MCC where the Elites speak in English. They have memorable quotes and make them far more expressive than in Halo CE, especially as allies.
-Hunters are once again bugged in Halo 3. This time they are extremely vulnerable to Spike and Plasma Grenades. A few well-placed sticks and they will die. Even moreso if you kill one of them the other will get insane and start lashing out without any strategy making it easier to get killed.
-Jackals are also bugged in Halo 3. Because the Plasma Pistol can now drain the battery while overcharging this change affected the Jackals hard. Most likely an oversight by Bungie themselves the Jackals will run out of Plasma Pistol battery if they keep overcharging the shot. When the overcharge light is off, you will get an harmless Jackal.
-Sniper Jackals now have lights on their heads, making it easier to spot them. Unlike Halo 2 they are also telegraphed to miss the first shot, giving the player enough time to react and act accordingly. (Thank God).
-Halo 3 and Halo 3 ODST feature the most well rounded Marine AI in the series making them less of a risky bet and more numerous thanks to the power of the original Xbox 360. However they are even worse against the Flood, Brute Shots, explosions and melee. Particularly thanks to the new mutation mechnics of the Flood.
-Brute Chieftains on Normal and lower are particularly vulnerable to the Radar Jammer. All you need to do is find one, throw it near a Brute Chieftain, then stick the Radar Jammer with either a Plasma or Spike Grenade and voila: for some reason the Brute Chieftain will get insta killed and never dodge the explosion, letting you grab his Gravity Hammer and Invencibility equipment for yourself. This does not work on any difficulty above Normal, as far as I am aware.
-Brute Chieftains can not be supercombined but they will still dodge Needler needles nonetheless. Use this to your advantage to disrupt their charging patterns by firing at them with a Needler then switching to another viable weapon to kill them easier.
-Support weapons (aka detached turrets) are useless on any dificulty above Normal because of the fact they will give the player a slow movement speed penalty.
-The Flame turret is by far the worst Support weapon in the game. You wil kill yourself from the burn effect and be roasted along with the enemies you were trying to kill with it. Unless you are experimenting do not use this Support Weapon. Ever.
-Halo 3 introduces the Suicide Grunt. This is a regular Grunt that instead of freaking out and fleeing after their squad leader dies will arm 2 Plasma Grenades and go kamikaze in an attempt to kill the player with them. They also have hillarious lines once they do this to let the player know their intention (because they speak English since Halo CE up to Halo 3 ODST).

Easy:
Easy on 3 is probably the easiest in the series. You could probably play this in your sleep. Still, it has some value to it. For example, if you're looking to just see what new weapons/vehicles are in Halo 3 and aren't looking to actually 'play' it yet, it could be smart to pop open a level on Easy and try to learn it's intricacies.

You may play Halo 3 on easy if:
-You only care about the story;
-You are new to video games as a whole;
-You are going for Par Time achievements.

Normal:
There isn't a whole lot to say about Normal on Halo 3, it's about as average as possible difficulty wise. For an experienced player it will likely prove too easy. Still, it's a fair pick for your first time playing Halo 3.

Normal on Halo 3 may be for you if:
-You are not new to video games but are new to Halo;
-You are going to play one of the default playlists;
-You hate Brute Chieftains as much of the rest the community does;
-You wish to get through levels somewhat more quickly.

Heroic:
Heroic is extremely well balanced on Halo 3. The game is still somewhat forgiving of mistakes but isn't too easy. Again, it's a fairly fun experience that is balanced in a way that makes you feel like a badass Spartan.
The Brutes are generally less difficult to deal with than the Elites and Brutes from Halo 2 by a wide margin, no longer being the bullet sponges they were previously. Some people believe they were nerfed too heavily. It's actually something of a contentious topic in the community. While I don't necessarily agree that they were nerfed too hard, I do agree with the idea that Brutes are less fun to fight than Elites.
Enemy weaknesses are a good knowledge to have but the game is a bit more forgiving on missteps than previous entries. Still, ranged precision weapons reign supreme in Halo 3 as they have in Halo 2.

Heroic on Halo 3 may be for you if:
-You are new to Halo but used to shooters;
-You are a returning player looking for a more casual experience;
-You wish to get achievements locked behing Heroic or above difficulty;
-You love Brute Chieftains and can tollerate a moderate amount of BS;
-You wish to play the game the way Bungie intended.

Legendary:
Bungie learned from the mistakes they made in Halo 2 and have crafted a more well-balanced Legendary in Halo 3 that is far fairer. Make no mistake, it will still be extremely difficult, but many of the issues that made Halo 2's Legendary unfun have been corrected. Iron is no longer forced on (Although MCC eventually allowed you to play 2's Legendary without it as well) and Jackal Snipers are hard-coded to miss the first shot and are much more visible.
Jackal Snipers now feature a purple helmet light that helps you easily identify them.
Legendary on Halo 3 is still less forgiving than Halo CE's and you will find that the TTK (Time to kill, a measure of how much continuous fire it takes to eliminate a player or enemy) is ridiculously low for you, with plasma pistols from grunts shredding you if you aren't careful. For that reason, taking cover in Halo 3's Legendary can be important in large swarms of enemies, and the aggressive style of gameplay Halo CE's Legendary promotes it at times impossible to recreate with Halo 3.
The biggest issue in Halo 3 Legendary in my experience is vehicular combat. Whenever a player enters a vehicle it turns the vehicle into a walking paper mache wit porcelain made armor and your Marines/Elite allies will be able to tank more damage than your character. It means you have to play hide and seek in vehicle sections in Halo 3 otherwise you will get melted by everything that is either on vehicle or on foot.
Legendary is very doable as long as you understand the Sandbox and enemy weaknesses to their full extent. It will take more effort but you can prevail if you put your knowledge to use.

You may choose Legendary in Halo 3 if:
-You are looking for a challenge;
-You love Brute Chieftains annoying the hell out of you;
-Actual vehicular warfare wasn't challenging enough for you;
-You wish to feel how saving Earth was probably like in first person prespective;
-You wish to get Legendary-exclusive achievements;
-You wish to get the last terminal's full message

ODST is a spinoff of Halo that puts you in the shoes of an ODST, the Rookie, in the streets of New Mombasa. Chronologically it occurs in the middle of the events of Halo 2, but I'd still recommend you play it right after 3 which was when it came out, as your knowledge of Halo 3's sandbox is essential to prevail.
ODST is a fun game but one that is quite different in gameplay. You will find yourself needing to employ more intelligent tactics and use cover in order to overcome your foes rather than relying on the sheer strength of a Spartan. Aggressive gameplay will generally not cut it unless you have mastered the movement and weapons to the extent you can overcome your reduced Health and lack of shields in order to push an advantage.
More than anything, the difference i'd note between Halo 3 and ODST can be boiled down to this: In Halo 3 you can rely on Master Chief's advantages to give you an edge in combat, but in ODST you need to focus in and utilize each weapon in the game's arsenal to the absolute best of your abilities on higher difficulties.

Some things to keep in mind about ODST on all difficulties:
-For some reason you can still rip turrets off their legs and carry them around like a Spartan, in fact you actually move faster than Chief with them. Congrats on the gains.
-You can still board vehicles as an ODST and actually destroy them with your bare hands. Just like a Spartan would do.
-The grenade throwing arc is different than the other games and will take some adjusting if you wish to consistently land sticks. This is because your character is no longer a Spartan but rather an ODST.
-Movement in general is different. Again because of the fact you play as an ODST and not a Spartan. This is most noticeably if you crouch and move at the same time, your weapon will slightly recoil as a reminder that you are an ODST.
-Your character still has 2 layers of protection. As mention before you no longer possess shields but have instead Stamina. Once you deplete your Stamina you start to loose Health. Stamina loss is communicated to the player via screen blur if your VISR is on and character pain noises, the latter is also a sound indicator of when you are taking health damage. Stamina will recover over time if you stay in cover or simply not take any damage for a certain period of time. However, ODST marks the return of the Health Pack as the only means to fully recover from Health loss.
-You can not use equipment but your enemies still can. Be extremely careful with this.
-Not every encounter in Mombasa streets is worth fighting with your limited health and supplies. Difficult encounters are therefore worth sneaking around whenever possible instead of fighting.Avoid hunters as much as you can in particular because of my next tip.
-Halo 3 ODST is the only game to feature 2 ranks for the Hunters. The standard blue hunter that behaves like the Halo 3 Hunter with its Fuel Rod Beam and the Gold Hunter. Equipped with a Fuel Rod cannon instead of a Fuel Rod Beam this particular Hunter will massacre you in seconds.
-Halo 3 ODST narrows the sandbox from Halo 3. You can no longer dual wield (well, you are an ODST after all) but you also have less options when it comes to a precision weapon jack of all trades weapon with only the Covenant Carbine being available. The Battle Rifle is only available in ODST Firefight this time. This means the carbine will become your workhorse in this game whenever you can get your hands on one. The 2 new additions to ODST are good for lower difficulties only (both the Silenced Magnum and Silenced SMG).
-Both the Silenced Magnum and Silenced SMG do not have a stealth trait. Unlike Halo CE Sniper Rifle, the moment you fire these weapons you will alert enemies to your presence. Both weapons are mostly used to dispatch weak enemies like Grunts or Jackals due to the low damage but high fire rate nature of them. The Silenced Magnum is headshot capable and marks the return of the scope the God Gun in Halo CE has, although its nowhere near as lethal as its Halo CE big brother. The Silenced SMG, on the other hand, functions more like an upgraded version of both the Halo 2 and Halo 3 SMG. It has a scope which makes it viable at medium ranges as well as close ranges. Besides these changes these weapons role on the sandbox remain very limited to weak enemies clearing.
-As teased before, ODST is the only game to not feature a flashlight of any kind. It shares a similarity with Halo Reach in that both games have an alternative mode of lighting available to player. In the case of ODST it is the VISR mode. VISR highlights enemies as red, friendlies as green and neutral elements as sillouettes (something like weapons on the ground or medkits).
-ODST is the only game to not feature a motion tracker in the MCC. So be very careful with this because its like having a permanent Fog skull placed on you (Fog Skull disables the motion tracker in the games it exists).

Easy:
Easy on ODST is just about as forgiving as it is in all other Halo games. In fact, if you play on Easy you'll practically feel like you're still playing a Spartan considering how helpless enemies will be against your unending onslaught. Brutes will fall before you by the handful and you'll at most come out of it with a few scratches.

You may play ODST on Easy if:
-You're new to video games;
-You only care about the story;
-You are going for Par Time achievements;

Normal:
Normal on ODST is average. It'll be fine if you understand concepts like occasionally taking cover, the fact you can't outmelee a brute at full health like you could in Halo 3, and you've hopefully learned the value of grenades.

You may choose Normal for ODST if:
-You're not too confident in your abilities;
-You're new to ODST but not Halo as a whole;
-You're playing one of the MCC playlists;
-You are still learning the tactics that make ODST differ from other franchise titles;
-Your knowledge of the weapons sandbox is incomplete.

Heroic:
Heroic. In ODST, from this difficulty up the differences between this game and Halo CE, 2, and 3 should be well understood by the player if you hope to do well.
For example, by now you should understand that your starting loadout's Automag is almost exclusively useful for trash like Grunts and Jackals. Exploit it's firerate and precision to pile on bodies and deprive Brutes of their companions, after which you can focus on taking them out individually with whatever other weapon you are carrying, or your grenades.
You should also by now understand that not every encounter is worth fighting through. On Mombasa Streets for example, flight is as good of a choice as fight if you find yourself low on ammo or in a position that is strategically unviable. If you don't have cover, don't reveal yourself. There's no shame in bypassing a Covenant patrol. You aren't the Chief and the UNSC aren't expecting you to win the war single handledly.
You should be used to using your VISR to check for Sniper Jackals along the rooftops and balconies by now, otherwise they will be an unpleasant surprise at several points in the game, though not nearly to the point Halo 2's were. The little bird raptor bastards still remain quite annoying.

You may choose Heroic on ODST if:
-You want to play the game the way it's meant to be played;
-You have gained total understanding of the mechanics of playing as an ODST;
-You are looking for an experience that is challenging but also somewhat forgiving;
-You're new to Halo but not new to Shooters.

Legendary:
What can I say other than ouch. You better be an expert of the new mechanics if you choose Legendary. If you make foolish mistakes like taking a low-firepower weapon into a gun fight, engage with a lack of cover or forget to throw grenades, you will find yourself dying again and again and again.
In spite of that, ODST on Legendary can be very fun if you HAVE mastered these mechanics as overcoming the enemy through sheer tactical prowess, overwhelming firepower, and strategy is a great hit of Dopamine. For example, using a sniper rifle to pluck off a Brute's support, then closing in and sidestepping him.
Generally, the best strategy for Legendary on ODST is, as self-explanatory as it sounds: Don't get hit. The longer you spend firing at enemies at times where they can't fire back, the better. Don't feel forced to rush, take things as methodically as you need to in order to ensure your tactical advantage.

You may choose ODST on Legendary if:
-You are a master of ODST's mechanics in every sense of the word;
-You wish to feel vulnerable;
-You are a veteran Halo player looking to style on some Covenant bastards;
-You have complete familiarity with Halo 3's weapons sandbox and can utilize the arsenal to its full potential;
-You are looking for a challenge that will test your ability to think quickly in tense situations.

Remember Reach, they said. And indeed I'm sure for anyone who played it, we all do. Reach is a bit of an odd one because much of the gameplay is quite different from any previous entries. Enemy behaviors are changed quite a bit, especially Elites which behave in a way we haven't seen before, or since.

Reach is a prequel to CE but I'd still recommend first time players play the games in the order of release rather than Chronological order.

Some things to keep in mind about Reach on all difficulties:
-Not every encounter is worth fighting for. There is no shame in bypassing any encounter if you can, especially against Hunters. This is because of the many buffs the Covenant received in Reach making them a pain to fight even on lower difficulties.
- In my opinion, Reach has the most balanced Plasma Pistol in the games on the MCC. Tracking is reliable, it does decent damage per shot, can EMP shields and vehicles and is still battery efficient in both overcharging mode depletion and after an overcharged shot.
-The Plasma Rifle is arguabely better in Reach than the new Plasma Repeater. It strips shields much faster than a Plasma Repeater. Funny enough the Plasma Repeater is God tier in the hands of the enemy AI.
-A common trend with Reach is weapons that are garbage in the hands of the player are God tier in the hands of the Enemy AI. Things like Concussion Rifles, Spikers or Plasma Repeaters will melt you in seconds if you underestimate them.
-Assasinating an enemy is faster than killing them from behind. This is due to the several buffs the Covenant received in Reach again. Only 3 enemy types can not be assassinated: Drones, Hunters and Brutes. Assasinations are an instant kill in any difficulty. However, the anmation will leave your character exposed so they must be used wisely. Reach and 4 are the only 2 games in MCC that feature animated assasinations.
-Reach melee system is the most broken in the Bungie games. Meaning unlike Halo 2 it is not dogeable and on anything above Normal it is a 1 hit kill.
-Reach brings back different health and shielding values across all enemy types that was lost since Halo CE. Like Halo CE every single enemy has different health and shield values on top of the difficulty modifiers.
-Reach is the other game where a flashlight is replaced but this time with Night Vision mode (hurray for Sam Fisher fans). Night Vision highlights silouettes but unlike the VISR it does not distinguish friend, foe or neutral.
-The DMR is your workhorse in this game along with the Plasma Pistol. The DMR has semi-auto 1 shot capability (15 rounds per magazine, plus 60 in reserve), unlike the 3 shot BR bursts so its the best UNSC weapon by far in Reach. The DMR is very ammo efficient aganst weak targets without shields like Grunts or Jackals.and fills the role of jack of all trades weapon really well. The Plasma Pistol is, of course, needed to strip shields and EMP vehicles. If a DMR is not available the closest alternative is the Covenant Needle Rifle that does supercombines as well but is semi-automatic instead of automatic like the Needler.
-You can not supercombine enemies in Reach that have no depleted shields. You need to deplete the shields first to be able to supercombine enemies once they are down to their health only.
-Hunters are still bugged in Reach, despite the many buffs. They can tank most weapons shots but are extremely vulnerable to Spartan Lasers this time. This is most noticeable on any difficulty below Normal. They can tank up to 1 rocket entire mag but they will get 1 shoted by the Spartan Laser.
-Reach has the most OP Sniper Rifle against vehicles by far. It really has AP rounds this time and you dont need to aim for the driver of a vehicle to get a kill on lower difficulties especially. Landing al 4 shots on the vehicle itself might just be enough to wreck it. Of course this depends on how well armored the vehicle is. Like if its a Ghost or a Wraith. This means the Sniper Rifle is an excellent tool to destroy shade turrets completely and not just kill the operator but make them unusable for other enemies completely.
-Reach introduces Reticle Bloom along with armor abilities. Reticle Bloom is divisive amongst the community but the fact is it makes most weapons behave extremely different than what they used to, especially precision weapons. It requires the player to pace their shots otherwise they will miss the shots the larger the center reticle gets. If pacing shots it ensures the center reticle size stays consistent and thus getting more accurate restults. Not firing the weapon will eventually reset the center reticle size. Not all weapons are affected by Reticle Bloom. Most notabely non-precision weapons and power weapons.
-Grunts no longer speak English but instead have their own dialect, unique to the Reach Grunts. This makes them far more unpredictable units on the battlefield.
-And last but certainly not least. I feel like I need to ellaborate on this. Never and I mean NEVER let a certain female Spartan with a bionic arm drive any kind of multi-manned vehicle. Its not enough that Halo has no seatbelts on any vehicles but this menace in light blue armor is bound to ruin your gameplay experience again and again. You are safer with a Trooper as your driver. Better yet I feel like I need to emphasize this if you somewhat are not looking and she manages to hijack your vehicle you are in for some fun and automotive carnage at the expense of the player and any Trooper unlucky enough to be in the same vehicle with her as the driver. Too bad you sadly do not have time to "fall in love with her voice" as she will likely drive said multi-manned vehicle of some random cliff, casing a milion dollar loss to the UNSC and personnel casualties that they so desperately need for the war.

Easy:
Easy on Reach is brain-dead levels of easy. It maintains the tradition of Halo on Easy being able to practically play itself. The main difference I will note compared to other Halo games on Easy is that first time players may still find themselves killed/caught off guard by suicide Grunts.

You may choose to play Reach on Easy if:
-You are new to video games;
-You only care about the story;
-You are going for Par Time achievements;
-You are trying to get that one achievement towards the end of the game that shall remain unnamed.

Normal:
Normal on Reach is pretty balanced. It's the last difficulty you can likely get away with using guns like the Assault Rifle and Plasma Repeater and the highest difficulty where it'll feel like the Marines (technically Army Troopers in Reach) around you are putting in any real work.
Normal on Reach is a decent experience.

You may choose to play Reach on Normal if:
-You're going through a default playlist;
-You're new to Halo but not shooters;
-You hate Reach Elites as much as 80% of the community does.

Heroic
Heroic on Reach is slightly harder than Heroic on Halo CE and Halo 3. You will find that Grunts suicide-charge you more with plasma grenades, Elites dodge your vehicle ramming/grenades more often, and Grunt Heavies and Ultras actually begin to pose a threat in numbers.
If you try to use weaker weapons such as the AR or Plasma Repeater on this difficulty you will find that you might as well be shooting at the enemy with airsoft guns because you're only going to get yourself killed.
On this difficulty and up you're going to carry a DMR or Needle Rifle on you at all times if you want this experience to be relatively painless. If you are the kind of person who thinks they're going to rush into combat with a plasma rifle and a concussion rifle you better either be a Halo veteran player or a recent Asylum escapee as it's probably not going to work out.

You may choose to play Halo Reach on Heroic if:
-You can tolerate a moderate but not overwhelming amount of BS;
-You have learned the mechanics well enough that you feel comfortable countering the Reach Elites;
-You have learned the mechanics well enough that you feel comfortable countering the Reach Suicide Grunts;
-You want to babysit Army Troopers constantly to ensure their survival;
-You want to play the game the way Bungie intended.

Legendary:
Reach on Legendary, what can I say. It's not quite Halo 2 levels of hair-greying madness but it can feel close.
Elites will have a tendency to dodge grenades they weren't even looking in the direction of and have shields powered by double the nuclear generators compared to Halo 2 and can survive at least 1 plasma stick for the higher ranks or multiple power weapon shots like multiple Rockets or Rods before falling, you will find that Grunts kill you fairly easily if you underestimate them, especially Suicide Grunts. Drones are the spawn of Satan and bringing any gun combo that isn't a DMR or Needle Rifle and a plasma weapon for dealing with shields means you might as well just jump off a cliff because you're going to die anyway.
Elites on Legendary in Reach are almost as frustrating as Jackal Snipers in Halo 2. Not quite there, but almost. You will find yourself emptying entire magazines of certain weapons into them only to find their shields are only about half-way down, you will find them making dodges that put Olympic runners to shame, and all the while they will lay fire on you with pinpoint accuracy. In the hands of the player the Concussion Rifle is practically worthless, in the hands of an Elite it's essentially broken in how strong and accurate it is.
You will watch legions of Army Troopers die without even killing a single enemy. You will see Suicide Grunt after Suicide Grunt charge you, and you will die. A lot.

You may choose Legendary on Reach if:
-You have mastered sidestepping Elites to the point you're comfortable doing this to large numbers of them under pressure;
-You don't like any gun that isn't the DMR or Needle Rifle;
-You have mastered countering the Suicide Grunts charges to the point you're comfortable doing this to large numbers of them under pressure;
-You want those spicy Legendary achievements;
-You want to feel what being on Reach probably canonically felt like.

Well, we now arrive at Halo 4. The black sheep of the MCC. This game was incredibly divisive at it's release and continues to be a subject of much contention. Mainly the criticism focuses around the story and I agree without a doubt that Halo 4 is well...not great. Though in my opinion it's also not as bad as many say it is.
In spite of that, you may still wish to play it if it strikes your fancy. While I'm not a huge fan of it's mechanical and visual changes there are undoubtedly some who are. For that reason I'm including it in this guide. Please do not turn the comment section into a fight club where you beat each other to death over whether or not you think Halo 4 is good.
As for me, I have a mixed opinion of Halo 4, I'm not a huge fan of it's art but I kind of enjoyed part of the story and, for example, some of the weapon rebalance that actually makes the Assault Rifle a viable weapon and I enjoy much of the arsenal's audio design.

Some things to keep in mind about 4 on all difficulties:
-Pitiful ammo capacities. Be prepared to suffer, especially the higher the difficulty is, of weapons with low reserve ammo without Famine skull (Famine lowers the weapon ammo dropped by the AI weapons) ever being enabled. You will turn into an experienced weapon scavenger by the end of Halo 4.
-Marine AI bug: When a marine on a turret kills an enemy they will fire at the corpse of said enemy for an unusual amount of time, leaving themselves and the player open for an easy death. Be very careful about this. It will happen and it will frustrate you. A lot. This is a bug derived from a previous Bungie era behaviour where the Marine AI used to shoot a corpse of a dead enemy with few rounds while yelling some random taunt line (which was funny to be honest and never got old, it gave them personality too and character). The enemies also did this behaviour in the Bungie games after killing either friendly AI or the player character.
-Most of Reach behaviours are here, albeit in weaker and more robotic ways. Elites melee is, as you guessed, still one hit undogeable kill on any difficulty higher than Normal. Much like the new Prometheans melee is. Although its worth noting in 4 Elite AI is in general dumber.
-Every single rank of Jackal and Grunt can now use the overcharged plasma pistol shot and they will do this very often. Contrary to Bungie games where only Jackal Majors and Reach Grunt Ultras could only ever do this. In addition to this, in 4 you can explode from supercombines from all enemies. Previously this only happened very sporadically during gameplay and usually only the corpse was supercombined after it was already dead. Not in Halo 4, in Halo 4 you can and will explode to instant death if enough Needles make contact with your body.
-Grunts no longer speak English but instead have their own dialect, unique to the Halo 4 Grunts. They speak in a different dialect than their Reach counterparts, however. This makes them far more unpredictable units on the battlefield.
-Halo 4 is the only game in the MCC that lacks the automatic discoping mechanic (this is a mechanic that automatically discopes a player that is using a scope on a scoped weapon if they are hit by a melee or ranged attack and was present in all the Bungie games). This means if you are scoping with a scoped weapon and get hit by any enemy attack, either being melee or ranged you will not discope automatically. This mechanic is extremely annoying, especially if you are scoping in aggressive enemies at higher difficulties where you can get a telegraphed melee attack that you could not see and because you can not use motion tracker while scoped (unless you are in multiplayer, with the awareness perk equipped) it can lead to some enfeuriating moments as your only clue is screen shaking when you get hit and not automatic discope. Be very minduful of this and do not forget to discope manually.
-Halo 4 has no lighting player assisted system of any kind. Funny enough in the teaser, your character was supposed to retain the flashlight but it never made it to the end game. Neither did a Night Vision or a VISR mode. This is the only game in the MCC that lacks this feature.

Easy:
Easy on Halo 4 is like ripping a bandaid off. It'll be quick and easy but still quite painful, just not due to it's difficulty. Enemies are weak and you slaughter them with the might of a thousand raging bolts of testosterone-fueled lightning even if you play with the skill of a blind deaf toddler with no arms.

You may play Halo 4 on Easy if:
-You want to get it over with as quickly as possible;
-You care about the story;
-You want Par Time achievements;
-You want to take out your anger on the art's redesign by bashing these ugly elites in uncontested.

Normal:
Normal is actually the difficulty I'd recommend most players play Halo 4 at. Not because Heroic or Legendary are super difficult, but for reasons I'll elaborate on in those sections. Normal is still relatively easy but enemies put up slightly more of a fight. The AI assisting you can still do a decent job holding their own when they aren't running headfirst into a wall repeatedly.
The main reason I recommend Normal for Halo 4 is that it can be mindless fun if you turn your brain off. It won't reach the level of greatness of other Halo games but can be a decent time sink if you just ignore the egregious story and hold down the fire button.

You may choose to play Halo 4 on Normal if:
-You feel like trying to actually enjoy yourself with Halo 4;
-You're playing through the default playlists;
-You want to constantly babysit Marines and Spartan IVs to ensure their survival;
-Your knowledge of weapon scaveging is incomplete;
-You want to experience Halo 4 the way 343i intended.

Heroic:
Well, here we are. Heroic. Halo 4's Heroic is best described as...tedious. It won't be insanely challenging in the conventional sense but Prometheans were seemingly designed with driving you slowly crazy as a key design element. Knights will teleport away right as you think you're about to take one down, Watchers will pop out of nowhere to catch and toss your grenades away, and overall you'll be questioning what you could be doing with your time instead.
You can get through Heroic on Halo 4 with time but the real question is why you'd bother.

You may play Halo 4 on Heroic if:
-You enjoy Prometheans annoying the hell out of you and can tolerate some amount of BS;
-You want achievements locked behind Heroic or higher;
-You want to feel what scaveging for weapons in a real war feels like.

Legendary:

Well. If you're seriously considering playing Halo 4 on Legendary I question your sanity. Will it be the hardest game on Legendary? No. Will it be worse than Halo 2 on Legendary in terms of difficulty? Absolutely not.
What it will be is mind bogglingly slow and tedious. Aggressive play is simultaneously a bad idea and the only way to really take on Knights. You'll have to meticulously clear each area one enemy at a time from behind cover, employing every cheap trick you can think of. Because the enemies will employ a high number of cheap tricks on you.
Knights will telegraph an instant kill melee that you can not dodge so enjoy being reset to last checkpoint by an instant kill attack you couldn't see or dodge.
Crawlers will eat up your shield in seconds with their fully automatic Forerunner BB guns in preparation for a Knight to trick shot you straight into Hell with their RGB gaming Forerunner DMR.
Incineration Cannon Knights will shoot with the accuracy and rage of Thor on steroids after you slept with his wife and will teleport away any time you're even close to killing them. They will also shoot the weapon without reloading.
Finishing a Knight on Legendary is more about luck, the luck that they either teleport somewhere that you can still hit them from rather than behind cover, or that their idiot AI just doesn't choose to teleport when their shields are down. Both can be quite rare.
Beyond that, vehicles might as well be made of paper on Legendary. You will struggle to drive five feet without hearing an enraging BEEP BEEP BEEP BEEP from your vehicle indicating it's about to crash and burn harder than Bitcoin did a few days ago.
Your AI companions may as well be toddlers with cap guns considering how useless their assistance will be. You'll watch Marines and Spartan IVs alike get decimated by any enemy, no matter whether it's a Grunt who is blind in one eye and armed with a stick shaped like a gun, or World Champion Heavy Weight boxer Hunters defending the title on Sunday. Either way your allies are as good as dead.
It's beatable, sure. But is it worth it?

You may choose Legendary on Halo 4 if:
-You enjoy Prometheans annoying the hell out of you;
-You have mastered the art of scaveging for weapons in every sense of the word;
-You want the Legendary achievements that 100% aren't worth it.

I enjoyed writing this guide, updating it based on a version from an older Steam guide. Of course I will keep updating this if I find the need to do so. Please kindly show your support to the original creator of the idea for this guide, and their guide, which I will link up here:

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2592533861

I hope you enjoyed reading my guide. Thank you.