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Answer from: Kirbone
To unlock the secret boss, you must first complete Main Mode with at least 70% completion to unlock Extra Mode. Complete Extra Mode to unlock the True Arena. Enter the True Arena and the secret boss will appear at round 13.
The secret boss is Galacta Knight
Note: He can be pretty difficult and has a wide range of attacks, not to mention he's hard to hit, recommended not to attempt him without a copy ability.