Inside the Earth Castle go to the second level and throw shurikens at his face. When he falls attack the blue orb that falls out of him. That is his weakness. Keep repeating this until he loses ALL his health.
At the Night castle there is one spot which has darts and super surikens. To reach it go to the right side of the jump point to get to the lever but instead of going up there turn left to find the ammo.
When you reach the location where there are trees and a man that will not leave until it is quiet, use the Temporary Control spell on the animal located on the map. Then go into the curve the animal is at and you will find a tunnel.