Unless i’m remembering wrong zoro did not land a clean hit on king without his flame before unlocking acoc, the only one we see is the clash with tiger hunt where zoro cut his mask but that is not exactly a clean hit, so it’s hard to say exactly how though he is, assuming he is like other dinosaurs, even with haki they all received damage from opponents that where weaker than zoro, and even the fodders of the bmp have access to the candy bullets that can easily pierce the germa exoskeleton, I doubt those would do no damage, now of course you could say that king has better haki that the other beast pirates, but even assuming his haki would completely block the damage, that would mean constantly coating, so that is another tax on his stamina, and as you said, haki runs out, there is 0% chance that he wins.