King couldn't do anything against KoH Zoro, pre-Advcoc Zoro almost fainted.
What he did against pre-KOH Zoro? I don't remember much but pushing him from Onigashima. I guess he half landed one Tempura Udon.
But still main issue was Enma and King's trick. Without Enma Zoro pre-KOH would have much easier fight and after learning King's tricky result would be the same just needed a couple of more hits.
Even with Enma result would be the same after learning King's tricky.
He was holding back with Enma, remember that.