You have to use Bowser to attack Dark Bowser to death until it dies, then Fawful revives him from within, then Dark Bowser becomes Bigger. You have to punch Dark Bowser's Belly, which he will spit Fawful. You have to use the Vacuum Block to inhale Fawful. Now, you have to use Mario and Luigi to punish Fawful to death, then attack the death star.
Imakehouseswithdirt
perfect!
I'm on a mission to spam peoples profiles.
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So much.
(Obligatory Gamechap Reference)