He didnt have trouble healing the cut from Tatsumaki, did he?
Hundreds of Luffy's hits are unable to leave permanent damage on Kaido's body. Digest that, slowly.
Kaido wont have time to feel that Zoro is a worthy opponent, death will be swift. One slice and head is gone.
We saw the gap in Zoro's favor when the mighty faced Hakai while the weakling was scared shitless.
What happens when he is done with Grip Reaper, who's next?
I will show you headless Kaido more than 3 times when the time comes.