No, I'm not disputing Diane's strength. Like I said, if she hits Kenpachi with anything significant - he is so dead. If Kenpachi does what he always does, which is dick around and deliberately take hits from his opponents before building his way up to serious moves, then he's going to lose pretty hard because Diane will mostly break him in one serious hit.
My problem is that Kenpachi, even being the comparative slowpoke that he is by the standards of Bleach, has speedfeats that make him way faster than anyone Diane's been shown to reliably tag. And he can fly so her ability to transmute the terrain is much less relevant to him.
So if Kenpachi being serious from the outset, which is the version I'm talking here, will immediately zip into point blank range and hit Diane with multiple
kendo level strikes which will hurt her a lot. If she tries to counter, he's fast enough to dodge her and keep getting hits.
So... my point is that Kenpachi has the stats to win this, he just needs to get his head in the game or he's dead.