He can't.
Why is this false narrative gaining traction?You realize that King was gracious enough to leave air and still bested him in Sword duel right?
After FCoC he started dominating in CqC
King is toying with him be it with flame or not like shown earlier when he couldn't take a breath from his vicious slashes .
Head on clash he stil lost when he had contact with King's body
King threw him around like a pinball and Zoro could do nothing about it.
All hypothetical scenarios with no substance.
It's no different than Luffy's fight with Katakuri where he apparently would be able to deal with his FS by eating away his haki,as if Luffy wouldn't ran out of his haki . We saw how close the fight was even after he learned FS. Yeah good luck.
If Zoro didn't have FCoC here
it was game over
Swords flying out of onigashima.
So no Zoro doesn't have answers to King without Fcoc.