Do the dungeon all over? I didn't think it was possible to run out of coins since the game is so generous with them xD

But yeah, that's one of the trickier fights. I usually start out by luring him towards stuff that's gonna explode in my face and then move away at just the right time. Once everything's all smashed up I go after him and that seems to work well enough. Also if you haven't been doing this already, always buy the equipment upgrades. They make a huge difference!