This achievement was glitched for me. I started a Mordecai run, and this achievement would NOT unlock. I ran over easily 100 enemies before I gave up on it.
If this glitch happens to you, your only chance is to create another character. Level that character up to 10 or so. Kill Bone Head, get the car system fixed just outside of Fyrestone, and then search around for any enemy that needs to stay off the road.
You don't need much speed to kill an enemy with the runners, so don't bother using the boost in the process.