Zoro told us in Chapter 1032 that he needs to find out King’s race to beat him.
In Chapter 1033 King receives further exposition on his race and what they are about by Queen, who responds to Zoro’s earlier claim that they would defeat the Calamities, rebuts that Zoro has no hope of defeating King, which adds further dread to Zoro’s situation.
The Lunarians were hailed as these gods who could survive in any harsh circumstances, and adapt to them. There’s still more to the Lunarians, as we do not know what exterminated them long ago, but King is a living example of what they are capable of. His ability to survive any condition is in full display as well, as he takes some of Zoro’s most potent attacks like nothing, and even survives a giant explosion he fashioned in order to destroy Zoro completely, while he naturally adapts to it with whatever allows him to.
Zoro cannot cut King, this is established. Queen said Lunarians can manipulate fire, right? While that info is correct, I’m gonna go ahead and guess it’s incomplete. They can manipulate fire because they ARE fire. Their bodies are, at least partially, made from fire. And who’s a swordsman we know that recently learned how to cut fire? That’s right!
Zoro only has to figure this out somehow. Once he uses the right technique (that he has already mastered), he wins. Of course, it might be more complicated than that. If it were that simple, even Kin’emon could beat King, which I don’t think is the case. I think strong Haki will be absolutely necessary. But my theory is that Zoro’s final attack on King will be Foxfire Style + Conqueror’s Haki.