Set the game to easy, play as a character, and keep walking to the right, save your special move for the boss, and use it on him, once he's dead and the game gives you the points and moves on to stage 2, just hit select on your controller, and choose to reset the game, then set the game to easy (it forgets this) and choose a second character, and do it again, reset the game, set the game to easy, and do it a third time with the last character, and the trophy will unlock.