Enma helped zoro to be stronger because he learned the advanced hakis, Enma is like hyogoro to luffy, once Zoro mastered her and understood, he doesn't need to use enma anymore, but before mastering her she was a nerf for zoro. So yes, Zoro doesn't need enma to harm kaido. Also the ashura he used, not only was it in its worst state, but it wasn't even after the PU. He did not consciously use the conqueror's haki, he did not overpower enma, there were no clouds on his swords and no black lightning