If you play through the game while killing and looking for everything, you will get this achievement. you will get it at about 3/4 in the game
But for people who wants to play stealthly, you maybe will need to play through the game twice (thats just good, cause it is a great game)
400 kills seems to be little, and it is. If you like this game and reconsider to play through it multiple times, then you will definetly get this achievement.