Kill 300 mutants.

Mutants will be present throughout the game and will typically attack you in groups. These are the first enemies you encounter during the opening level, Moscow and come in various shapes and sizes depending on which level you are on. It's highly likely you will unlock this achievement through natural play but if you need to grind it out you can load any level, find a group of mutants, make a quick save, kill them, then reload your save. Repeat as necessary.