BSB tips:
Never, ever corner it. You can't dodge around a cornered BSB!! I've seen so many people die because they got too aggressive and kept attacking while it was cornered. Calm down, back up, let him come out into the open. The beast actually has horrible aim! As long as you...
...Never stand in front of it's face. Ever. BSB's "safe zone" is absolutely anywhere but directly in front of it. Doesn't matter how far away it is. You ideally want to face either of the skin flaps hanging from its sides.
In his third phase he'll do that charge up attack, right? If you're close you can knock him out of it with a heavy. Good opportunity for a visc.
You can summon an NPC hunter next to the stairs leading down and it's an active coop area to boot so help is fairly plentiful.
A torch can very briefly interrupt some of his lighter swings. He can be parried easily once you've got his timing down, too. But that's a lot of trial and error.
Funnily enough, almost all of these tips go for VA as well, as I found out last night when I slaughtered her.
Will have to bear in mind when i finally come up against BSB...