Killing enemies through fences
Use the following trick to kill enemies without taking damage. When in a level with fences, such as at the Prisons, stand behind one with your unlimited long-range weapon and fire through. If done correctly, you should be able to kill anything you see through the fence. You can also walk through a fence. Once on the other side, wait until zombies or other enemy appears, then and run back through the door. Fire at them until they are dead.