King is not weak, Lol. Being able to stall Top Tiers is a quite the feat, not just anyone can do it.
And of course Luffy is still landing the final blow. Zoro will either reopen the scar left by Oden, or create a new one of his own. But the final KO attack will be reserved for Luffy. If the fight stays in the sky on Onigashima, and the threat of it landing will kill hundreds of thousands of citizens, I can see Luffy's last attack that KOes Kaido, also destroys the entire Island. Kind of what Luffy tried to do with the Noah, but succeeds with Onigashima.