Damage the following Bosses with the indicated technique. A sound will confirm correct code entry. Defeat the boss, then the next level will then be in Hard mode.

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