The difficulty of Kizaru is not damaging him, it's catching him and being able to even hit him in the first place, that's why conqueror's coating doesn't matter that much, though I don't understand why Luffy didn't use it when he could hit Kizaru in Snakeman (yonko nerf???).
The moment Luffy can do that, I think it'll be over, but will it happen or are we simply being teased by Oda?