Kill 100,004 Zombies

The strategy I use is to use a rollerhawg and go to Ingelton (top left city) in the later chapters. The streets are usually packed with zombies and there aren't many roadblocks. All you have to do is drive around and span the fire attack. You're also going to want the upgrade that makes vehicles invincible so your hawg doesn't get destroyed. The zombie respawns are very quick so you don't have to worry about waiting for more to come. This achievement also carries through to different save, so you can do this in nightmare mode also.