You have to beat adventure on VEREYHARD mode on one life{ you must do this for classic as well}with everyone. once done, you will face every single one like if you were to gain luigi, ganondorf or somthing like that. you unlock all stages if you beat all hidden people. IT WORKS I TRIED IT AND SEVEN OF MY FRIENDS HAVE TRIED IT. GGGOOODDD LLLUUUUCCCCKKK!!! !!!