Yeah. A duel. Cuz it worked out so well last time. Wait, when was last time again? Oh, literally 2 minutes ago? Old man, go get yourself checked, you might have a concussion.
Unless he is finding a justification to go against tradition by losing her to the Demon King, then he is stupid. You can't even beat him with your whole group attacking him, what do you think will happen on a 1v1?