So it’s not that Luffy became more durable, he started mitigating the damage from attacks by coating himself with ACoC… This explanation makes sense, and it once again proves why the Admirals are so afraid of Conqueror’s Haki users — after all, they have no way to mitigate the damage. Basically, the hit that an ACoC user lands on them is going to kill them, while ACoC vs ACoC is much more fair. LMAO
That’s why a fresh Kizaru ends up crying and shaking on the ground, unable to get up after a single basic COA blow.