Medusa

When you get the boomerang on your way to Medusa, KEEP IT! Don't pick up any other side arms. It's a lot easier to finish her with the boomerang because it her hits her, keeps going, AND returns to you only if you do it close enough. You'll quickly find the rhythm on how to do it.

Level 9 trick

In level 9 ONLY, jump on the same treasure chest 255 times to recieve a free power-up!