Just make a game with 4 teams, an put your three opponents on an allied team. This way they won't kill each other. To optomise the opportunity to get the bezerker do this too:

Put it on:
Play for fun
Full wormage
Verticle landscape
0 minute round time (sudden death), so everyone has a 1 health
indestructable terrain
no mines or barrels

I just got it in a fair game.

Much easier done on the PSP version, unfortunately. Probably the easiest way to get it legitimately on the DS version is with a similar custom scheme, but with lightning strikes, no sudden death (just start with 1 HP) or hazards of any kind, and a very stupid opponent.