Ultimate Chicken Horse

This is a cumulative total and must be done in actual games (can NOT be done in Free Play). If you do this in online games, only kills with your actual account will register, but if you play local couch co-op with a shared controller, all kills count since all characters are playing on the same account. You should work on other achievements first, since you'll likely make progress towards this, but when it's time to grind this out, the fastest way is to create a local shared controller game with four players, change the round limit to 25 rounds or whatever the highest is, have the first player place a trap right in front of the starting point, and then have every player run into it at the start of every round. You'll be able to get 3 kills per round (1 is a suicide technically), so this will go by pretty quick.