Can’t Yujiro Udonde into Benda? Or any move for that matter, as all Undonde is is appearing to have an open stance, baiting your opponent, and then attacking. It’s never specified that he has to use a certain attack (like how Cockroach Tackle has been an finger breaker, a tackle, and a three punch combo).
While I'm unconvinced Hanma's attacks will inflict immediate physical damage to Luffy, pain manipulation would likely result in a KO.
He has the AP to hurt Luffy over time, but put Pain Manip via Benda into the mix, and Luffy taking a hit would result in him taking a lot of mental trauma in live combat, which will likely distract and lower his fighting efficiency.
Ironically, Hanma's attacks would be resisted by Luffy's rubber body, but that's not enough to secure Luffy a W, and simply because Hanma has various other attack and especially defensive abilities, thus he easily has more ways he can win.