Here's a video guide, but for a written guide:
1. Go to Mount Kastka and enter the castle.
2. You'll have to make some tough jumps with the up attack ability - beware!
3. Grab the side dash ability.
4. Fight the boss.
- This boss will shoot chains of ground level fireballs, as well as random raining down fireballs, as well as emitting small eyeball enemies.
- Dodge the ground level fireballs, stay close, and attack the exposed eye.
- Dodge the raining fireballs - I never successfully did once, though I never died.
- Kill the small eyes and quickly attack the main eyeball.