More info based on Korean spoiler:
Zoro tells Law that this attack is the best he can do so if this doesn't work, it's up to him.
Zoro chants some dialogue with the Asura technique (provider omits some of the wording)
Zoro attacks Kaido with it.
Kaido: "You also have King's haki?" (Some dialogue omitted by the provider).
It's no surprise that this attack will leave a wound.
Kaido attacks Law with Thunder Bagua