That was maybe the toughest guy in ToB..
Maybe cheesing with summons can help out not to soak the damage with your characters.
There was this really nice sword called Carsomyr, and it had a special ability to dispel when it hits.. I dont recall if it worked against that dragon, but i think theres a good chance it did. So each time you hit him, he looses his protections and stuff..
You need a paladin to equip it but the sword is so damn good that its worth it.. you can even upgrade it to +6.
Btw versatile party with druids, thieves, bards is the most fun to play.
But if youre going for power builds,from my experience nothing beats brute force for boss fights, not even casters.
I had a paladin, kensai, berserker, fighter and sarevok.. had one support character but cant remember who.
The party was like... hacks, wacks, choppin' that meat