Go through King K.
In the Glitz Pit, become a fighter, after you beat the Goomba Bros. Reserve a match,(your fight is against the KP Koopas) King K. Will come up to you and say that your next match is against him. He'll stand in the doorway, go behind him, and when the gaurd opens the door,and there you have it, you go through King K:D
Note: This works only in Chapter 3.
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