All you need to do to get this achievement is play through the game. It is the first of four killing zombies in numbers typed achievements and by far the easiest. I have not gone out of my way to get this achievement and I have it, that is how easy it is to attain. Although I have said this you could just go out and kill zombies instead of doing the story, if you do this then you could double up on other achievements i.e. by making different weapons to do so.