Q)What is Boss Rush Mode?
A)Although I haven't unlocked Boss Rush Mode, By the name of it, I can give a few things about it:
1)You have to have 100% complete to unlock it.
2)It is a mode where you battle bosses (like you really did to get pieces of the Amazing Mirror).
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Q)What is the Power Center?
A) The Power Center is a room that has every Power except "MASTER" and "UFO." People say that you must open every door to every area to unlock THE POWER CENTER. Well they forgot something! To unlock The POWER CENTER, You must Not only need open all the doors to each and every area, BUT You must also defeat the Radish Ruins BOSS (Dark Meta Knight). After you beat Dark Meta Knight and open every door to every area (not counting the mirror that takes you to the last part), you will see an animated screen where a door is appearing (Starting from the top, and ending at the bottom) at the top-left part of the HUB. When you return to the HUB (MAIN ROOM;Area 1, Where you start), you wiil notice that the mirror that appeared on the animated screen will actually be there (IN THE EXACT SAME SPOT IT APPEARED).