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.