How do you defeat Dark Yojimbo?

[Heath]

Just summon Yojimbo and get him to do Zanmato.

Don't that just kind of destroy the point of looking for a challenge?

I suppose it does, but it's how I got rid of some annoying Dark Aeons in my first playthrough when I couldn't be bothered training to have everyone strong enough to beat them. A bit boring, but it gets them out the way.