Easiest way is to start working on your power and agility challenges, you need to do these for the i'm a runner and a fighter achievement anyway. Go to Buckshot's power challenge in the tower(first safe zone), his first challenge will involve killing with a bat. Go do the next closest power or agility challenge to unlock Buckshot's second challenge. It involves killing 30 infected with throwing stars, make sure to stop at 29 kills so you don't win the challenge. Replay it and finish the full 30 kills and unlock the achievement.