Left 4 Dead

Start a new game in "Dead Air" under the Easy difficulty and select the finale. Once you have made it to the tanker, restock on ammo, grab a pipe bomb or Molotov cocktail, then answer the radio and start the gas pump. Once the pump is activated, run around to the right-hand side of the rescue plane and prepare to fight off the horde. A benefit of this position is that the horde cannot come at you from behind. When it is time for the Tank to appear, it will usually spawn on the far side of the plane. Run up to opposite side then run back and forth without coming around the plane to your side. Your AI teammates will not shoot the Tank when it is on the other side of the plane. By crouching you will be able to easily aim at the lower body from beneath the plane and take it down quickly.

Select the Easy difficulty while playing offline. Let the bots clean up the enemies on the map. If you get healed, you can restart the map after dying and not have it count against you.

Enlist the help of another player. Start up the "Crash Course" finale as an Infected and progress until the end. Do not kill any of the bots before the doors open (when the second Tank is killed). Have the second player remain outside the exit, spawn in with your new Tank, and kill the survivors. The three bots will not enter the vehicle unless a human player does.

• If you have a health pack and are low on health, pause game play and select "Take A Break". While you are idle you will be healed. When it is done, rejoin the game and the healing will not count against you. Note: This was done under the Easy difficulty setting.
  

• Play on the No Mercy campaign under the on Easy difficulty setting. Take a MedKit at the beginning. You now have two choices. Either run off the ledge behind you (hold the Analog-stick Up and you will land on something above the dumpster), or go through the apartment. If you run off, you will be at 64 health (do not jump off or you will be at 2 health). Go through the campaign as usual after that. When your health is in the low 20s, your teammates will try to heal you. Take out your MedKit and they will stop. You should be incapacitated somewhere in the subway or sewer level. After this, grab every pack of pain pills find. If there are multiples of them, grab one, use it, then grab the other. If your teammates have pain pills and they take them out to give to you, do not take out your MedKit or they will put them away and try to heal you for about twenty minutes. When your health is in the low 70s, your teammates will try to heal you. When you are in the finale, there probably will not be any pain pills. If you find some, it will be due completely to luck. Search everywhere on the roof and inside. Break open one of the doors below the door that leads to the Jesus Room. Do not try to open the door to the Jesus Room because you will die trying. Then, contact the helicopter. Run for the room you selected and stay there, MedKit out. Your teammates should follow you inside and will keep you covered. Do not worry about the Tank; your teammates will go out to fight it. When the helicopter arrives, you will be at 1 health. Moe to the stairs that lead to the helipad; not the ramp or you will die. You should be incapacitated when you reach the top. Keep the Infected off you as best as you can, then run for the helicopter. Do not worry about getting hit; if you do not stop you will make it. Because you did not use your MedKit, did not allow your teammates to heal you, and relied only on pain pills (which do not count as healing), you will unlock the "Unbreakable" achievement.
  

If you have a health pack and are low on health, pause the game, and select "Take A Break". While you are idle, you will be healed. Once you are healed, rejoin the game, and the healing will not count against you. Note: This was done on the Easy difficulty.

There is a glitch will allow a friend to invite you into the last chapter of any episode (as long as it is from the Dashboard). Completing it from that chapter will make the game flag that you have gone through the entire campaign and not just that last chapter. This also works for all other achievements related to doing a "whole campaign", such as "Safety First" and "Upset Stomach". Only people invited in from the Dashboard will receive the achievements, and not the person inviting.

• This can be done anywhere, except in Versus mode. You must kill 53,595 zombies. When playing in Campaign mode, you get a few friends (Louis, Bill, Zoey, and Francis) to help you. The bots can sometimes kill the zombies and special Infected more than you can. In order to kill more zombies than the bots, find a sniper rifle. Get to a high area and snipe each zombie. You can also go lower, but turn off the flashlight or your snipe can be ruined by the attack of a zombie. The best levels to do this are any until the finale in "Blood Harvest"; "The Greenhouse" and "Crane" levels in "Dead Air"; "The Church" and "The Town" in "Death Toll"; and the first three levels in "No Mercy".
  
• Start the No Mercy finale, with bots under the hardest difficulty setting. Kill the bots and vote the difficulty down. Leave the safe room and run through the large corridor until you find a Boomer. If needed, reload; do not climb up. After getting a Boomer lure him back to the safe room. Enter it and lock the Boomer outside. If done correctly, you should be standing inside the safe room and he will be standing outside in front of the door. Hold [Melee] while standing and aiming at the door. The Boomer will now vomit at you repeatedly. Zombies will spawn and you will melee them to death. Remain idle in this state by mechanically keeping [Melee] held.

The Survivors must jump into a sewer hole at a certain point. As an Infected, use this to your advantage. While playing as a Smoker or Hunter, hide behind one of the large canisters in the room. Once one of the Survivors open the hole and are done checking the vicinity, they will jump in. Due to the fact they must jump in one at a time or get stuck, make sure you have a clear view of the last one in line. Once all three are down and the fourth Survivor is still up, pounce or grab him or her. By doing this, he or she will not be able to be saved, unless someone throws a Molotov cocktail or pipe bomb.

This trick works best as a Hunter. Before you spawn in the No Mercy campaign, Level 3, go all the way to the very end, at the hospital. Once up the ladder, turn to your right, and there should be a dumpster. Move it all the way over to the manhole, and one of two things will happen. It will either fall all the way down, which is bad because the Survivors can't move it; or it will partially fall in. The survivors cannot move this at all. If you are a Tank, you can move a car over it, which is better than the dumpster. From here, do not puke on them as a Boomer, as it will attract the Horde. The Horde will try to move the object away from the manhole. Then, attack them as Hunters and Smokers. They may shoot you, but every so often, you can hurt them until finally they die.

Before you spawn, go all the way up to floor 28, where the elevator lets out. Just outside of the elevator is a generator. Move it in front of the elevator door. If the Survivors know what you are doing, they can move it. It requires three survivors to move an object. However, if you have someone else on the Infected side, they can go a little bit further and move the forklift in front of the doorway.