Hmm Akainu is a good candidate durability wise I would suggest, mostly in that he took 2 island splitters in all likelihood without channeling a barrier and merely suffered some moderate damage. I think those attacks are potentially not weaker than Current Luffy's KKG given how even Hybrid Kaido's Thunder Bagua looks extensively weaker than those Gura strikes.
In chapter 838, Cracker put a full weight slash from the side onto an extended arm using his DF to trick Luffy's arm into such a prone position.
That is not effortless, that is a full power sneak attack slash that didn't breach bone or inhibit Luffy's offense.
It matters because without bizarre sneak attacks Luffy's arm has never been slashed prone like that, even against the worlds strongest swordsman... Now Luffy has FS he can't be sneak attacked without PIS which isn't relevant for power scaling.
Still it doesn't change the fact that, Luffy can be cut even in G4 mode or any stronger mode by a proficient swordsman with a renowned sword irrespective of whether it is a sneak attack or Luffy can avoid with a FS.
I also rely on this argument a lot.
Boro Breath managed to cover G4's face in blood. By consequence I don't even want to think what Hiryuu Kaen or Asura could inflict to Luffy. Not even Tankman is a guaranteee things would substantially change.