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The Secret Boss is accessed via the Mystery Coin password. The button is "hidden" somewhere on the Minigame Select Screen.
Rank Achieved by Reached
1 KC ManagraKC Managra (playing Drag Box) 1 times
2 KoGKoG 1 times
3 SakuraSakura 1 times
4 xxxxxx 1 times
5 DanarothDanaroth 1 times