Best level for this is the airport.
Get those leaning bitches shot up into the helicopter blades at the bottom right of the level. I like to stand against the wall on the right, right inside of the helicopter blade. The blades can't hurt you so don't worry about taking a wrong turn somewhere.
As the zombies approach, I just shoot from top to bottom, and the leaning zombie bitches will jump right into the blades. This got me a TON of free lives on the 7 days of hell mode since I scored almost 500 hazard kills on the level.