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Go ahead! Embrace the cheapness! It's the only way! Remember: sometimes, the genius of cheapness is more valuable than skill, for even though it won't get you many admirers, it will get you more trophies!

You can unlock Boss Battle Mode by clearing Subspace Emissary Mode. I know very little about this mode but I can imagine that it will probably be just like All-Star Mode, only with bosses instead of normal characters. I will post more when I have better information to give.