Here's how to get this achievement without any possibility of failing:
During Episode 3-2 when you get to the Coal Mine Museum, a short way in you will go through a door and enter a building into a long hallway. Three taken will drop down from the ceiling and a fourth will walk in from the left. Toss out a flashbang toward the group and you will get all four of them without fail.
Note: Reloading the checkpoint will put you back right outside the door you just walked through. See my other solutions for getting your 50 flashbang, 50 flare gun, and 20 Indirect kills while you are here.
http://www.youtube.com/watch?feature=player_profilepage&...