Find the Zombrex in the back of the ambulance. You will get a cart with half of a bike. Push it out and back to the safe house. After the cinematic, instead of bringing that cart directly back, keep pushing it around into large groups of zombies. The cart never brakes and a few hits will kills almost every zombie (except for SoldierZom). Note: Start at about 8 a.m. and complete at 1 p.m. for best results.