I was fairly high level when I killed the dragon (16 or so), but here is what I did. I had the wardstone, so I had Jaheira summon a fire elemental. He killed this with a death spell, but did not go hostile. So I attacked him and Edwin (best mage in the game, let no one tell you otherwise) let loose with a spell sequencer of 1 Greater Malison and 2 Ice Storms. Everyone was hitting him, casting lots of Magic Missiles and then Aerie and jaheira cast Doom on him, lowering his saves even further. Edwin finished the job with a Finger of Death.
He has the Crom Faeyr scroll, that alone makes killing him worth it. (and a little XP, too, but you know, who needs that?)
