Death Proof is a simple but tedious achievement that just requires you to die 100 times. It is not hard to do, but it may take a while depending on how often you play. You should be dying normally as you play the game and face various enemies and bosses. The best way to get this achievement is to run into enemies as much as possible in the lower levels, and if you have a lot of soul points, use the revives, as getting knocked out counts as a death. Another easy way to do it is to have a friend play with you and revive you as many times as possible.