This is a story-related achievement and cannot be missed. It will unlock upon defeating Killabilly, the final boss of the game. Defeating Killabilly is simply enough. He'll start out by grabbing you and you'll need to use your Chainsaw Blaster to shoot him in the face. After his first phase focus on sawing his fingers off when he lays his hands on the ground. He'll conjure up zombies in-between attacks so be sure to dodge those and slay any zombies in your way. Once you have him at "finish" he'll trap you with his hand. Perform the quick-time event to finish him off for good.