1- Luffy always outshine everyone against the main villain. This is a recurring theme in One Piece, with the Strawhats including Zoro always leaving the main bad guy to Luffy
2- Zoro knows that for Luffy to become pirate king, he needs to defeat the yonko/admirals, and the reason why Zoro trained was to help Luffy accomplishing that dream
3- It has already been confirmed for years that Luffy will be the one to defeat Kaido
From logic and common sense alone then, it's already obvious who will defeat Kaido
Zoro will likely be among the people who will ultimately weaken Kaido, but Luffy will be the one to finish him in a 1 on 1