Here's a guide, but for a written walkthrough:
1. Go below Darsov to the middle of Valeshire, and enter the castle.
2. Grab the Thunderstorm ability while you're at it!
3. Fight the boss and win.
- The boss has a fire attack that hits the floating platforms, and the center of the fight area. He also beams himself forward, and shoots out attacking eyes. Crouching right in front of him will avoid all damage.
- Jump up twice and hit him in the head after one of his attacks, or 3 times if you're quick enough.
- Take your time, and the boss battle is yours!