I don't think Law has to take away Zoro's swords just to beat him but for someone as skilled as Mihawk it won't work. Mihawk can harm Law when he activates his room, much like we've seen Vergo do in the past. Also, didn't Law take his heart back when Vergo was holding onto it? What's stopping him from taking it away from Zoro? It could just be an inconsistency on Oda's part why he didn't take away Fujitora's swords.