F.E.A.R. 3

Do split screen co op and at the very beginning of INTERVAL 3 to the right there is a barrel that is knocked over and there are flames right next to it...just use your dummy controller and let him go in the flames and keep getting downed...then you keep reviving him until you get 50 revives with your main controller...let me know if this helped guys :-)

Edited: "Just remember to put it on insane difficulty so you die faster, and player 1 can melee/kick/slide player 2 to speed it up."
**Credit to TheRealPH in the Comments**

All these solutions work but I think the fastest way to do this is to start a split screen co op with a dummy controller and load interval 08 on insane. The pistol has unlimited ammo so u can head shot the dummy controller right at the intro of the level. It takes a lot longer to melee them than it does to dump a whole clip in their head. Took me about 7 minutes.

I didn't have enough coop play so I did the following. Start a game, level one on insane split screen coop. I used point man. Four jump kicks and one slash of the knife and down goes Fettel. Revive. Rinse repeat. No enemies will bother you and won't take more that 15 to 20 minutes to do all 50 once you get your rhythm down.

When your teammate is in last stand you have to go over to him and revive him. You must do this a total of 50 times in any game mode. If you want, you can down your own teammate over and over until you get the achievement but this should be unlocked through natural progression if you play co-op enough.

I've made this video, and it's pretty easy to do!
You can do this with two controllers or with a partner but start Interval 6 - Bridge up, kill all the demon creatures down the trains and when you move up, you will counter some enemies. Just kill them, but when you move ahead to the big control house and up the stairs and down to the ground. Then you will see a lot of enemies spawning and a "Blue Man" with a laser. Move the one who has to get the achievement to point in the video, and the one that will be downed all the time at the other point as you can see in the video and just let the enemies shoot him, and you standing behind him. This is a really easy method.