King at his maxiumum defense is far far far more durbale than Kaido to the point where one of Zoro's weakest attack does this to Kaido
But now Zoro has Advanced Conqueror's Haki something that boosts your AP so much that where Luffy's CoA punch did zero absolutely zero damage to Kaido Luffy's CoC Punch did this to Kaido
Imagine how much stronger Zoro's advCoC Attacks are when he can already do damage to Kaido without CoC so Zoro's non CoC Offense >>> Luffy's non CoC Offense, Zoro's CoC Offense >>> Luffy's CoC Offense
yet King tanks two of Zoro's CoC attacks at his least durable form
Considering King can tank even Zoro's ridiculously strong CoC attacks, he'd be tanking Luffy's CoC attacks for hours.
Granted in the end Zoro defeated him with such a strong attack even while he was in fire mode he got his wings cut. Zoro destroyed all of King's durability but I guarantee you that attack will kill even Kaido.