This can be combo'd with Walk-Over. Both can be done with one person, one controller.
Simply go to play, party battle, single controller, and fill up all ten slots with the generic avatars they've provided (or custom ones if you have them, doesn't matter). Complete the game, and it'll pop at the Rematch/Main Menu screen during the credits.
To combine with Walk-Over, simply pick one family side and have them win every round plus fast money.