So, question. After Zoro gets healed and has his fixed time limit to fight and defeat King. How's he going to kill Kaido later down the line after getting hit with double the effects of Hakai, and whatever damage King ends up doing to him?
Kaido will be killed by combining large amounts of CoA haki with adv CoC. This is what Oda has been working towards. Those are two power that aren't affected as much by physical strength or body condition. We saw when Zoro first used enma how powerful an attack can be when using large amounts of haki. Thanks to the large amount of haki released from his body he cut a cliff using only the physical strength needed to cut a tree.
I believe one purpose to Zoro fighting King is to help him improve his CoC.
The manga implied that his flying dragon blaze attack, which had a huge amount or overflowing amount of CoA haki, would have scarred Kaido. That's why they were concerned about the attack.
We know adv CoC with normal be amount of haki can scar Kaido. Oden scarred Kaido without using large amounts of haki.
Kaido has some of the best recovery and healing powers in the manga. He hasn't even use DF awakening which should further enhance that power. I believe that is the reason why it's nearly impossible to kill him, not beat him. Even if he's defeated he's going to continue to return so to currently remove him from Wano he must be killed. The best way to kill him is with a single attack powerful enough to take them out.