... a mysterious girl shows up (obviously Akane) and they interact, talk, catch up etc. The perfected version of the Umisenken basically renders him able to efface himself from reality and become untouchable unless he wishes it. The girl (transparently Akane) took the opposite tack and developed a technique that anchors her so deeply in reality that she becomes more real than anything around her; not untouchable but indestructible.
It might have been a oneshot. I think they're the only two characters who appear in the whole thing. Ranma is some kind of legendary hermit. Anyone seen this one? My google-fu is strong but I'm stumped.