I would keeping update this guide in future, basic just written down some thing I seen in this serial of the game.

Halo-1

• you can't pick up energy sword yet
  
  
• you can't switch gun with company yet.
  
  
• Plasma pistol could disable the energy shied.
  
  
• the pistol own the best damage and fire rate thought this game.
  
  
• the shotgun are op, not just the high damage and the large magazine.
  
  
• master chief and arbiter both crush the ship in the game(halo1, 2)
  
  
• a lot of freedom in the battle, you could even speed run skip most of the battle.

Halo-2

• rifle shot in triple shoot (36 rounds)
  
  
• you could one shot kill the first boss with energy sword.
  
  
• the start of the duel weapon system.
  
  
• from the halo-2 developer remove the hp bar.
  
  
• the flash light die out like five second.

Halo-3

• first time work with the flood(like one minute then went riot)
  
  
• you could run over the little brush or tree in the road.
  
  
• pistol work like crap in halo-3(low fire rate and magazine)
  
  
• old game got four grenades, from 3 you could own three type of grenade but two per maximum.
  
  
• tank can't shoot the machine gun round by yourself now, you need a company to do that.
  
  
• nerf down the damage and magazine of the shogun, now this weapon are terrible to use.

Halo-4

• autosetry are the best, don't go other.
  
  
• had the most difficult level due to the lowest ammo thought out the serial, most of time you can't even fight just because there you run out everything in your hand.
  
  
• assault rifle only reversed 224 rounds backup ammo(600 rounds in old serial)

Halo-reach

• noble six the green warrior are present as the master chief.
  
  
• only 2 grenades now in two type.
  
  
• rifle are semi auto now(15 rounds)

fight or flight
fight or run it on you mind, did run if possible to skip unnecessary fight if you could do that.

vehicle and turret
vehicle and turret are you best friends, not just the firepower their own and the infinite supply they got and remember to grab the machine gun with you.

rush zone
all you want in high difficult are pushing to next save spot so you could process the story, do remember this.

batter safe than sorry
pushing slow if you found difficult, stick with AI company if possible, they would die quick if nobody assist.

used all you got and thing on ground
do remember used all resource that you got in the road, especially the weapon and grenades on the ground, you couldn't only carry two weapon and some grenade other thing would be wasted if you didn't used.

took out the weaker one
take down weak enemy first, so you won't wasted ammo on the shield.

don't mess with the big one
enemy like hunter need too many resource to take down, do evade if possible.

punch it
punch and shoot at same target to maximum your damage output.

jump for it
some area content too many enemy but you could evade by jumping on platform.

two gun is batter than one
switch to second gun is always batter and faster then reload

let them come
stand at a high ground which let you take out enemy one by one as them come

flush them out
by used the grenade you could flush out hiding enemy

take a rest
don't stress yourself not always easy to fight in higher difficult so take some break after you playing from level.

maybe.... restart?
if all the option above not helping, then maybe you had too low hp or ammo or even save at weird spot, so the only one are replay the scene, I known this sound suck but it's the final option.