User blog comment:Godisme/Everything But The Rain/@comment-5606268-20130227161448/@comment-393772-20130228022351

The only problem is that it wasn't stated WHAT exactly is was that kicked Isshin's ass.

He says, and I quote, "How'd you kill that by yourself...?! You're not just some ordinary girl, are you?"

Therefore there is no plot hole at this point.