Small Arms

Start up Lord Pete's Mission Mode, once you get to level two, do not move from where you spawn at. Shoot right-downward towards the log. The enemies will always stay there, just keep shooting them using the secondary weapon occasionally(LT). You also have ammo and food spawning on top of you.

Right, as cheap as this sounds, start up a multiplayer game and sign in with your profile on controller one and a 'dummy' profile with controller two.

Now choose a level where the opponents can fall out of the arena. (I chose the new ice berg level from the downloadable content.)

The aim of this is to punch your 'opponent' off the arena a total of five times in quick succession. Shift your dummy opponent with the second controller to the edge / gaping big hole of the arena and then smack your opponent off with your profile character!

When they regenerate above you, make sure you move the control stick of the dummy controller so they fall off they're invincibility platform and in to your grasp sooner to keep the combo going.

Easy!

My personal experience was creating an xbox live battle and waiting ten minutes for no one to appear so I signed in with the second controller. On the Iceberg level, you can punch your opponent off the boat to the left and for some strange reason they keep respawning on the top of the middle part of the level so you can keep punching them to the right and into the drink!!!

Within a minuite, that's achievement unlocked!

This is very easy to do even if you don't have a second controller. What I did was to choose Battle Mode, then my opponent were ISO-7982 (The orange robot) and you choose Tyrone then you choose 5 lives (instead of 1 that's shown in the video) from rules and set the level to Bazaar. When you start, wait until ISO gets to the bottom floor, and then you're gonna jump to the right and hold Y a second and aim up to the roof on the right building, wait until ISO is midair and then press LT to freeze him, make sure you don't stay to far away. Then when he dies, jump down to the floor again and repeat until the match is over. I got this achievement in 30 seconds with this method. Here's a video that shows how you should do it.