"This achievement tend to glitch, so here is what you want to do (easy way). Simply play the game as usual and kill people, don't bother sparing them all, just play as normal. After you complete the game, when you get the ending and then the game backs you to main menu. Go to your Xbox360 settings, storage, hard drive then to section where your Metro Last Light saves are stored and there delete PREFERENCES. Then come back to game and start a game from chapter select 'D6'. simply complete the level killing ONLY the FLAMETHROWER guy and wait till end. And here you go, a cheevo for you. Easy and 100% working. Hope it helps."