Extra Stage Boss songs:
Select Game mode. and clear a song under the Expert difficulty with at least an "AA" rank on your last stage. This unlocks the Extra Stage, with settings at 1.5x Speed, Reverse Scroll, and Rainbow Arrows. Depending on which song sort chosen when the game was started, one of the following Boss song will be selectable. Choose that Boss song and clear it with at least an "A" rank to unlock it in the shop for purchase.