BB gets his has handed to him using haki. He biggest asset is yami-yami no mi which can disable devil fruit power. However using haki stops his power before it stops yours. As witnessed during Marineford. White Beard was kicking his ass using haki, like he kicked Akainu's. With sufficient haki level Luffy would win. BB knows about haki but I don't think he can use it. Even if he could if Luffy teams up with Law to stop BB Law could just cut through the Haki. I honest feel like BB is the weakest Yonko.