Actually since I posted, I have actually beat it.
The trick is you you chain together, a whole heap of quickening, and cast Berserk on vosler, and then just attack all out.
P.S: All my characters were level 22.
Exactly.[/Christopher Lambert Raiden voice]
To the topic creator, if you're stuck on Belias, make sure to set up a Handkerchief gambit while fighting him. His big attack causes Oil (and I think his normal hits do to *can't remember*).