Successfully complete the indicated task to unlock the corresponding type of challenge in Challenge mode:

Easy Boss challenges: Reach the Boss on the Easy difficulty.
Easy Mini Boss challenges: Reach the Mini Boss on the Easy or Normal difficulty.
Normal Boss challenges: Reach the Boss on the Normal difficulty.
Normal Mini Boss challenges: Reach the Mini Boss on the Normal difficulty.
Hard Boss challenges: Reach the Boss on the Hard difficulty.
Hard Mini Boss challenges: Reach the Mini Boss on the Hard difficulty.