[Annheiser]

I'm genuinely surprised you're having that much trouble killing the Black Dragon when you've gone through the Red and Shadow ones. o.0 Have you tried summoning a few monsters (Bears, Elementals would be great)? You should send your melee classes in first to draw its attention, then send in the monsters afterwards. Hit it with debuff spells (Breach, Pierce, whatever). The only thing really annoying about the dragon is its area tangleroot spell but at that point you should have high enough saves to ignore it.