As soon as you start the level, get on the bike nearby (press E on it). Then keep running enemies over until you get the achievement. Your speed has to be fast enough to kill them on impact, so make sure to back away from them a bit before you run them over. You can take the bike into the underground sewer with you, but I don't know if it is possible to drive it out from there. That's where I got the trophy. And remember, you're not invincible on the bike, so set the game difficulty to Super Easy.