Left 4 Dead 2

Ok, so this is very self explainitory, but getting it is a bit difficult. So if you wanna get this in just a normal versus mode, the door of the safe room has to be closed with at least one living and one dead survivor outside of it. If the door does not close, then you cannot get this.
If you want the easy way of getting this, boot up versus with one friend (or just an empty second controller b/c honestly they are not needed for anything,) and find a defibrillater. (Easiest place I have found is the very first chapter of the very first level. There is usually one in a hotel room or a storage room that you have to go through.) Once you have the defibrillater, go to the end of the level, but dont go in the saferoom yet. Knock all your AI teamates health to next to nothing then, repeatedly knock one of them down until they die. Now, this is important, knock the other two down, but leave them alive. Now, go into the safe room, shut the door, open it, run out a difibrillate the dead teamate. Don't worry if it doesnt pop right then, just revive your teamates and go into the safe room. Mine didn't pop until after I had finished the level.
Any questions or suggestions, comment.

Edit: As stated in several of the comments below you need to have the infected knock down and kill the person being revived.

you don't have to close the door with two controllers. simply kill 2 AI partners off at the beginning. then move through the campaign, once you reach the safe room enter the safe room and then step outside so the computer will follow.

in my case, the computer didn't follow, but I spawned my hunter on my 2nd controller, pounced him in the safe room, then defibrilated him before killing the hunter. the achievement popped once he was revived.

Hello achievement hunters! This solution is pretty simple the defibrillator is usually in the ambulance on that level if not you can find it on the table of the "free garage sale".

Aura's solution above works, but here is a step by step of what i did after several failed attempts. Also please note that this method gives you the achievement offline only, if getting it online is important for you, look elsewhere.

This solution will use two controllers, on versus.

1- Clear your cache. An update from Valve added special infected bots, they will make this hard/impossible.
2- Load up Dead Center/Hotel. Make sure you select a human character so that controller 2 gets the infected.
3- Play through the level, picking up a defib unit on your way. Might take two or three runs, its appearance is random.
4- Once you get everyone inside the safe room, kill two bots. Make sure that the remaining bot is visible from the outside of the room so that a smoker can get a shot on it and pull it out of the room. You want that bot to have a good amount of health, heal him if necessary.
5- Using controller 2, let the bots kill you and respawn until you are a smoker.
6- With the smoker outside the safe room, grab the surviving bot from inside the safe room.
7- While the bot is being hacked at by the smoker, have your achievement seeker close the door for a few seconds. **Reopen door before bots dies**
8- Once the bot is dead, go out and defib him. Achievement popped right there for me.
9- Lesson here is, smoking kills ;)

Hope this helps someone!

(you will need two controllers for this, unless you have a friend)
So, load up dead center chapter 1 and make your second controller the infected, go through the level normally and make sure you pick up a defib, once you reach the end, DO NOT enter the saferoom, instead close the saferoom door from the outside, this stops the bots from running inside, it also forces the special infected to spawn inside the saferoom and they can't get out.
Then you can sit in any Corner outside the saferoom, (when you're facing the saferoom, I sat in the left corner)
Use your second controller to kill one bot, and use your main controller to incap the other two bots, then shoot through the saferoom door, killing the special infected, quickly run in, close the door, then open the door, revive the dead bot and finish the level,
You should get the achievement,

Also when it shows incoming attacks, no zombies will spawn.

I don't know if this is a very consistent method, it worked 2/2 times I tried

Good luck guys 👍