Light my fire Achievement
Light my fire20
Set 10 zombies on fire simultaneously.
Using a Molotov is likely your best bet here. Throwing a meat bait out will easily group them and keep them off you if you have that as well. During act two you can exit the church and see an ambulance on a bridge to the right. Head across it and aggro those zombies and the few at the end of the bridge as well. Run back and stand on top of the ambulance to get a group of ten, then drop your Molotov on top of them.
There are multiple other places throughout the city to get this. Use a pistol from far off to aggro zombies and jump onto a car or truck (truck is best). You can run around the area quickly to find more then go back. Shoot anything in the distance so that they come to you.