Zoro's own strength + Enma with Oden's haki inside + Asura (Zoro's strongest technique), all these factors play a role. Enma does play great role but discounting Zoro's own strength is not right, Kaidou was in hybrid when Zoro scarred him while Kaidou was in dragon form when Oden scarred him, so Zoro using his 100% definitely played a part.