Well, for Xbox 360, there is no cage match anymore
But all I'd suggest doing is being a dick with Marathon, Lightweight and Commando, find a place on a big map where someone is with an assault rifle, run across, wait to regenerate health, repeat.
hurry up and do it in a cage match. ill let u get it if u dont have anyone else to help. just get shot in the leg once and u could let the other person know when your health is fully recovered so they could 4 more times.
If you can't do it in a cage match then jump into a Free For All (FFA) with a mate. Get him to shoot you in the leg and wait for your health to regenerate each time, you'll get it done in no time.