Grab an Olympia and shoot them in the chest, head or legs.

If you shoot them in the legs, it should blow their legs off and they have to crawl on the ground which makes them easy to kill. Shooting them in the chest doesn't always work but it does help.

As you already know, as you get farther and farther, they get harder and harder to kill so when you get into round five or six, it would be smarter to shoot them in the head.