This is the new mainline game, not the fighting game.
It's got 4 playables: Armpits, Thieving Black and White, Luigi and Second-Best Rabbit
Demi is stage 1-3 afaik. Seems to have discarded lives for instant kills, but you can start over as many times as you want from different waypoints/chapters while retaining score.
So basically, you just keep playing until you've cleared the game with no hits taken.