He Was A Skater BoyG 25

Easiest done in Time Trial mode. Load up Town, race 3. You'll see him take off while you're waiting to start. Once you start driving, you'll probably find him face down against a wall on the left (he sucks). Wait for him to get up and continue, then run him over/melee him. He'll eventually get up and continue again. Repeat 4 more times, and the achievement will pop on the 5th knockdown.