Volke

When you're fighting the monkey chief, you have to roll into one of the pillars he's standing on as he pauses to jeer at you. This should knock him off. He should be lying on the floor stunned at this point, so run over to him and slash his buttocks as many times as you can with your swords before he gets up. From there, repeat the process about two or three times and the Gale Boomerang is yours.