I had always three scenarios on my mind. The first one is that Zoro and Luffy are doing a combo attack and beat Kaido. The second one is that Zoro gives kaido a deep wound and Luffy punches with a strong Ryou Attack on the same place where the open wound is and Kaido gets beaten. Luffy participating on the final blow has one problem even though he is the MC, Kaido needs to die and Luffy would never kill Kaido. After Wano there’s no role for Kaido, he’s to strong , you can’t send him to Impel Down, he would break out easily. He can’t return during the final war, he will never be on Luffys side and I doubt that he will return as a villain again. So I came to the conclusion that he needs to die and my third scenario is that Zoro kills him. Maybe Luffy will " beat " Kaido, he’s for a short time K.O and then he goes desperately into the Dragonform with his last strength and then Zoro chops his head off.