Well, I don't remember the specifics of the fight, but paralyzing blood mages is generally the best option. Try to use Petrify or something that can stun them... the blood wound spell they use is almost instant death, no matter what... :/
Make sure you stun them every chance you get, and kill them first, no matter how many lesser enemies are around. AoE attacks like Tempest can be very useful...
If he's using that lame-ass leech aura that seems to leech 10% of all your party member's hp every second (ridiculous) then you just need to chain interrupts/stuns or use a mage character to dispel him.
Typical Blood Mage fight, he just has more health. If you aren't stunning him then you NEED to move out of the bad stuff as fast as possible. Ignore the Shades until he is dead.