In order to not make your time with this game any more difficult then it will be, there are a few things you should know and do when you play Cover Fire.

• You don't need to run the game to wait out the times for different things.
  
  
• Don't forget about your free crates as you can only wait for one at the time. They are essentials!
  
  
• Play the campaign mainly and try to get all objectives in your first try as you will get the most amount of rewards per level and also at the end of the chapters. Getting all objectives on each level is not mandatory for any of the achievements.
  
  
• It's impossible to get all stars in Chapter 13, due to one level does not count the headshots objective at all.
  
  
• After you beat a level for the first time you can replay it for 3 more times, after that you need to replenish the tries with gold. However, every 8 hours you get one retry back so you can wait that out.
  
  
• Each level has a combat level requirement and if you don't have enough, the enemies are going to do more damage and have more health as well. You could still beat levels which has higher combat level, in general levels from Chapter 9 and earlier can be beaten even if the difficulty is on extreme, but not higher than that.
  
  
• You should upgrade the cheapest costing skill to have a good amount of combat level, no matter what the skill is. If you need cards to upgrade a skill, click on that skill and press the info button to see what levels can drop those cards and replay it if needed. Farm upgrade cards on levels which require higher combat level, those drop more skill cards then the lower ones.
  
  
• The battle (or skirmish) game mode is allegedly the multiplayer game mode (mostly with bots), in which if you win games you can get additional rewards like perks, gold and even gold cards. However, there is a chance that the mode is going to be stuck in the player searching state for you and you can't play it, however if it's working for you, you should farm this mode whenever you can.
  
  
• Do headshots, as they do the most amount of damage.
  
  
• A decent way to get gold is to complete daily challenges, complete events, free crates and level up by XP.
  
  
• Best way to get money is to beat new levels and replay the boss levels and complete all of the objectives.
  
  
• When you see an event open, play it and complete it fully to get a new character, weapon and some gold, too.
  
  
• Use gold to open crates only. If you need perks get the big crate for 35 gold which also gives you a chance to get gold cards. If you need gold cards only, buy the epic crate, even though it's more expensive.

The only way to really endure this game is to "manipulate time" and get some of the rewards earlier, replenish the energy and tickets faster, get upgrades sooner and reset the daily challenges if you pass the time to the next day. With this method you can't "open up" battle/skirmish mode, but you can play event chapters earlier than you are supposed to.

I learned about this method from another guide, made by Berzerk Wrath, thank you for finding it.

It's important to note that this method has the potential to corrupt your save file or make your game behave unusually, so you should be mindful. I don't take any responsibility for any kind of loss.

You can change your system’s clock time and go back to the game, then the time will progress in the game. Once you get what you need, you should leave the game and change the time back to the current one to avoid any potential issues. Of course, when you get back to the game some of the rewards will have more time to come, but you can use the time manipulation method again to get them faster.

Safest thing to do is to not change your time by more than 10 hours and don't use this method more than a few times. Always make sure you change back you computer's time to the current one and play missions in that.

You can get the achievements in whatever order you want, but I recommend doing them the following way.

The beginning...
You have to complete 3 objectives in a mission to perfect it, which should be a piece of cake in one of the early levels.

3 better than 2
You have to kill 3 enemies with one shot. The first time you can do this is in Chapter 1 Mission 6, where if you wait for the 3 soldiers behind the cover to be aligned properly, you can kill them with a shot.

Not a good sign...
You have to lose a mission, which you should get naturally.

Shooter / List of Target / On Strike / Almost there... / Bounty Hunter
You have to kill 50, 100, 250, 500 and 1000 enemies, which you should get naturally.

Critical Eye / All the heads / Head Hunter / Losing the head / Big-Head
You have to perform 50, 100, 250, 500 and 1000 headshots. You should keep doing this throughout your playthrough so you won't have to worry about it later.

They have arrived / Rush hour train / Al-Shaddar / Welcome to the jungle / (Un)hide laboratory / Frost road / The man on the high castle / Awakening / Eve / Passengers to the train / In waves / High flights / Odyssey in space
You have to complete all 13 chapters in the campaign to get the corresponding achievements. This will take awhile, but once again you will get this naturally.

You are the good one, dont you?
You have to kill a civillian, which you can do in the first few levels.

The way to perfection
You have to upgrade a skill 10 times, which you will do anyway in your playthrough.

Collector
You have to get 200 cards, which can be either a skill, character or weapon card. You will get this naturally.

Stomache
You have to do 100 shots to the stomach based on the description, however you need to kill 100 enemies with stomach shots. Once again, you should get this one naturally but if the level doesn't require you to do headshots, you can do these instead. If you have some old levels to playthrough, you can farm this there.

Butter hands
You have to do 100 arms shoot, which similarly to the last achievement, you have to kill 100 enemies with arm shot kills. You should get this one naturally in your playthrough, but don't forget about it. If you have some old levels to playthrough, you can farm this there.

I can not feel my legs
You need to do 100 leg shots, which is similar to the other two achievements before this, so this mean you have to kill 100 enemies with leg shots, however this description is incorrect because for whatever reason you need to do 250 kills, not 100. You should farm this out in older missions.

Mercenary
You need to be level 15. You get XP from completing missions, winning battle/skirmish mode and complete daily challenges, so if you keep playing the game you will get this one, although it will take some time for sure.

Fast ball / Doctor's prescription / Critical time
You have to use 25 of each perk throughout the game. You can use perks with the 1,2 and 3 buttons. The most difficult part is actually have 25 of each of them, as specific perks only gets dropped in specific boss missions, also you could get random perks from crates or you can win some in Battle mode (if it works at all), or buy perk packs from the shop for real money. The most difficult one to obtain is the slowmo perk, which is obtainable from crates or from event mission rewards.

If battle mode is not working for you, this is going to take quite a lot of time.

Ace of Spades
You need to get 100 golden cards, which are the character and weapon cards. You can get these from events and from opening either the big or epic crate, but I recommend opening the epic crate. Once again, this is gonna take a lot of time to get it out of the way, but eventually you will get it.

If I were a rich man...
You need to have 250 000 money at once. You should work on this one when you are around the end of the campaign or you already beated it, as you will need the money to upgrade your combat level so there is no point saving up until then.

You should replay boss missions or high level normal missions, do daily challenges, complete events, wait out the free crates to get the amount. Also, you could buy the money from the shop for around 4€ which should instantly unlock the achievement, if you don't have patience to get the amount.

This takes a lot of time to complete and this seems just doable with using the "time manipulation" once again.

The Resistance / Gun-lover
You have to get 10 characters and weapons from the golden cards. Base characters and weapons DOES count to the achievement.

Even though you can get a character and weapon after you beat an event completely, this are still the hardest achievements of the game, because there are only 3 events available in the PC version, so you still need to open enough cards for each 3 more characters and weapons from crates. This takes stupid amount of time and luck to obtain it, so don't lose hope.

Use 'time manipulation', otherwise this achievement can take months to get it.

If you want to check your progression in the achievements, go to Cover Fire's achievement list on SteamDB[steamdb.info], and if you scroll down you can find a bunch of stats being counted in the game, most of them are related to the achievements.

In the campaign, at the end of each chapter there is a boss level, which some of them have some difficulty in, so I decided to give a quick rundown about each of them.

Chapter 1: Hell's square
You have to destroy a mech at the town's square, meanwhile it moves around and a few soldiers attacks you.
You have to first destroy two of its gun parts, then shoot the middle part. Don't attack when it shoots you. When your cover is destroyed, move to a new one with the A or D button. Make sure to kill some of the soldiers too.

Chapter 2: Unexpected machinist
You have to kill a minigun wielding enemy.
Just damage the boss until it moves away, then deal with his minions. After a few screens you can finish him. Don't try to fight him when he shots you back.

Chapter 3: Sand and fire
You have to destroy a spider-like tank which has a shield on.
When the tank fires its laser the shield goes down. Change your cover and start attacking the laser cannon part.
After the cannon is destroyed, the tank loses its shield completely and you have to destroy the legs. It will occasionally shoot back with a machine gun, which you can endure behind your cover. Don't forget the attacking soldiers either.

Chapter 4: Gate keepers
Simply kill all enemies and drones in different sizes. Don't shoot the drones when they are firing back at you. Take your time with this level.

Chapter 5: Darkest Day
After you go down in an elevator to a spacious room and kill some soldiers along the way, you will face the mutant.
You just have to shoot it until it dies. Shoot the gas canisters when it throws it at you. There are limited number of soldiers on the level, so you can also take them out first, then deal with the mutant.

Chapter 6: Frostbite
The level is similar to the Chapter 2 boss fight, where you need to damage the boss and kill all enemies to proceed to the next scene, but this time the sniper will move to a position which you need to find, the game will highlight it for you if you are not in scope mode.

Chapter 7: Psycho Soldier
At the start of the level you can't damage the psycho as it has a shield. When the psycho extends its arm, it moves gas canisters close to him to attack you, and you have to shot one of these canisters closely around him, which makes him lose his shield and that's when you can damage him. Do this until you defeat him.

Chapter 8: Sentinel
When you are in the circle shaped room, you will see a laser turret (Sentinel) which will soon target you, you have to damage the gun part first. However, if it's ready to attack you, you have to press either the D or A button to get to a different cover in order to dodge the attack to avoid getting damaged.
When the turret is badly damaged, you have to hit the center compartment, right next to a one big circle. When it's ready to fire, don't shoot at it, as the laser will hit you if you are standing.
Then you have to destroy another turret and two more center compartments at the same time. You can move to other covers this time. If you take out the turret first you should have an easy time until the end.

Chapter 9: The Tunnel
This is a car level and you have to shoot down the attacking enemies. The gimmick here is to stay down when a sign appears, to avoid getting hit and some of your health. Kill everyone and survive the ride.

Chapter 10: The Phantom
From this point on, you should have exactly the amount of combat level or more to attempt a boss level or have a different character / weapon which has better stats, because it gets significantly difficult now.

You have to kill a teleporting soldier who also has a shield.
Similarly to the boss from Chapter 3, when the soldier ready to fire the shield goes down, and this is when you can damage him. After each attack, the soldier teleports to a random location, meanwhile you also get attacked by regular soldiers and robots too. You can also change your position between three covers.
At the beginning, you should kill the first soldier on the left then start doing damage to the boss. Make sure to hit him immedietly when the shield is down to stun lock him, with that he won't fire his weapon too much. When there are more soldiers attacking you, you should take them out first then focus on the boss again. If you have slowmotion perk available, use that when the boss is ready to fire on you and input as much damage as you can.

Chapter 11: Bazooka
You have to destroy a helicopter and a guy on it.
The boss himself is pretty easy, he occasionally shoots rockets at you which you can destroy, meanwhile you will get soldiers, robots and zombies from the building ahead of you. This is where the level gets more difficult.
You should pay attention to robots walking out from the building, as they don't have a shield immedietly so you can shot them down fast enough. Soldiers and zombies are not that important, make sure they don't swarm you. In the meantime, be aware where the helicopter is and shot the rocket down when you get the warning about it. You need to micromanage these variables well here and it's recommended to upgrade your character higher then the required combat level to make it easier.
Also, there is an objective to kill all 8 enemies on the level, but there are 14 enemies to kill so don't get surprised if they are keep coming.

Chapter 12: Wasp's nest
You have to get through a robot wasp nest.
To reach this nest you have to deactivate multiple force fields by shooting buttons on the wall, meanwhile soldiers and wasps attack you.
Best way to deal with this level is shooting the buttons as soon as you can, make sure to take out enemies who you can reach first without dealing with the buttons. Try to minimize health loss until you reach the nest.
When you reach the nest, that's the harder part. It's highly recommended to use grenades here if you can, as you need to take out a lot of targets. You must take out as many wasps as you can as quickly as possible. Spray and pray through the wasps and watch out for the occasionally attack from both sides of the room.

Chapter 13: Monkey Wars
In the final boss level you have to kill... a bunch of space gorillas (sounds like a pretty crazy plot).
These gorillas can make holograms from themselves which can damage you, so in each room you have to find the real one and damage it as much as you can until it dies, and you have to do this in 3 rooms and each room increases the number of holograms. It's pretty much random which gorilla is the real one, and if you damaged the real gorilla it will change it's position and have to look for it again.
There is no way to figure out which is the real gorilla and I couldn't really figure out a strategy other than tough it out. This is by far the hardest level of the game, you should over level your combat level and get some perks. Health can keep you alive, granades can sweep through some of the holograms in one of the wave and slowmo can give you some time to find the real enemy. Use these to get through this level.