BB is strong but don't you think you are underestimating the SH crew.
We still aren't sure he has a Haki or not.
BB has Gura Gura and Yami Yami no mi and no Haki is needed to hit BB so i think the whole crew is more than enough to defeat him.
I also think Luffy could give him a tough time even though it is a one on one battle though the whole crew strength of BB maybe more stronger than that of SH.

people keep overestimating them, he's had 2 years to enhance his abilties and he apparently might have gained some more, he probably has come up with away to stop the main weakness of his DF, extra pain, by utilizing the other abilities he has, if luffy and co. were yonkou level before they even enter the NW, or even close to that, it would really kill the story, they'd be able to walk through it with ease, the whole point of OP is that they're challenged, they don't just one move pwn everyone and get instant powerups like in bleach.
Bb could probably beat them with ease, he'd probably take a couple hits and then own them.