Tell your partner to go prone in a corner, then sprint and jump on top of the guy in prone and tell him to stand up. *This may take a couple tries but with probably work* You will be basically stuck inside each other. The zombies wont be able to hit you and eventually they will die automatically. But be careful, throwing grenades is dangerous as it may ricochet around inside of the glitch, killing the person who threw it. (If I remember correctly, this glitch only works until about round 30. (One or two zombies may hit you but wont do anymore damage, if they continue to do so, you probably didnt do the glitch right.)