It's been a while since I read up on Casshern, I actually used him in a small gauntlet against Luffy (along with Cyborg 009, and Karas) trying to gauge Luffys strength awhile back on here. Cassherns regeneration I could see posing a problem for Luffy (I mean he basicaly TRIED TO DIE in Casshern Sins, he wanted to die and he couldn't..If I recall correctly there were a few times he let people beat the holy hell out of him where he wanted to just die but his regen wouldn't allow it and since he's doesn't age he just kept living on.), the only problem like..we never really see Casshern challenged..I mean Dio did hold his own (I was secretly rooting for Dio...he was my favorite character besides Dune). Luffy is incredibly fast, but Casshern is no slouch I'd say Hypersonic+ when he snaps and goes all out. Although Luffy I would say definitely has a larger pool of attacks...I don't really know to be honest. I'll have to think about this more.