This achievement is fairly easy. Join in with a second controller, and create an arena match on a level like the clouds level, where there are a lot of edges, and set points to 5.
Then when the game starts, with the second controller, go towards the edge, then move the first controller (your character) next to the second player. Press B to push them off. Then just run in front of the first player again and repeat the same thing, pressing B to push the second player off. The achievement will unlock after the game is over.
NOTE: if you even hit once, or use any weapon, this will not unlock.