Metro Last Light

There isn't much to this achievement. As far as anyone can tell, you cannot restart checkpoints/chapters to kill the same enemies over and over until you reach 100, so this has to be done naturally over the course of the game. If you kill every human enemy you come across (including reinforcements brought in from alarms being sounded off) starting in the Pavel chapter, you will have 100 kills by the end of the Revolution chapter.

Unfortunately you cannot reset the game from a checkpoint and kill all enemies that you have already killed, but it is not something to worry about because there are several achievements in which you kill a certain amount of enemies, and after you finish the campaign it is you are likely to play in Hard mode.