In the water treatment plant, right after Rebecca takes the lift downstairs, when she changes the camera angle, she can lure the zombies back to the area near the lift. However, the zombies will not be able to enter the camera frame close to the lift. She can stand at that position and continue to knife the two zombies without getting injured or hit even once. Similarly, in the steam room in the water treatment plant with the lift, position Billy or Rebecca near the lift facing toward the screen. Have your character stand next to the dangling florescent tube and lure the zombies to come from the door. The zombies will get stuck at the corner of the railing and can be stabbed until they are all dead. This helps to conserve ammo.