Grab a second controller and load up the mini games menu. Set up a match in online and public boxes ticker. Join the lobby with your profile (the one you want to get the achievement) and the one on your second controller.
Ready up by hitting
as soon as you can (before any other players join). Then just beat the crap out of whoever you want to get the achievement with.
Remember to keep an eye on the hearts and let your profile regenerate health so you don't kill them by accident. Using your fists helps avoid accidental death.