At the beggining of the duel, the large Migen will punch downward at you. You can grab upwrd, and catch his fist. Once you have his fist, you can throw it at the large Migen, but the smaller one will heal him before you can defeat him, so it's not worth the effort. Instead, throw his fist at the smaller one. When he gets a baffled look on his face, you can catch his next punch, and throw it up at his face. Continue this pattern until you beat him. (He will begin going faster, howeever.) Also, be aware for his fake punches, where he won't actually punch at all - Just attempt to fake you out.