A quick trip to the east side of Leisure Park will uncover both a parking lot and the entrance to the maintenance tunnels. Jump in the car sitting in the parking lot and head into the tunnels to rack up mad zombie kills. They're wall-to-wall in those tunnels, and your car will run down anything it touches.
The car deteriorates quickly, though. Head to the east while in the tunnels to find another car parked at one dead end. Use that car to head back to the original parking lot, where your original ride will have respawned. Repeat this process and you'll easily earn achievements like Zombie Hunter, Zombie Killer and Zombie Genocider (that's over 53,000 kills!).