Damage the following Bosses with the indicated technique. If done
correctly, you will hear a sound. Defeat the boss, and the next level
will then be in Hard mode.

Axel Gear: Have him get hit by the tracking purple ball attack
that he uses.
Wolf King: Wait for him to stand on top of a breakable floor
block. Hit him until laughs, then drill attack the floor so he falls.
Wolf Robot: Reflect his dynamite back at his face.