The last level of Sky Castle will be a boss fight against Master Mittens. This is a very simple boss fight.
First, Mittens will send a group of enemies at you. Simply hit them with your spin attack to send them flying to their death. After that, you’ll receive a health pickup and will then need to dodge fireballs. Again, you’ll receive more health pickups after. Mittens will then send one enemy at you. Hit the enemy towards Mittens to damage him. Dodge some more fireballs and repeat with the next couple of enemies. One more set of this and you’ve successfully defeated Master Mittens.