The problem is that the dragon starts off with about 39,000 hit points and regenerates about 200 hit points per round, so you have to do him well in excess of 600 hit points per round, or it's going to take so long that your party will die first. You need a fully promoted party to take this monster. (Doubtless somebody will prove me wrong on that, but it's certainly going to be a lot easier if your party is fully promoted.) The spell that really hurts him is Poison Cloud. Get close in, look down, and cast the spell on the foot that is closest to you. That way, the Cloud hangs around and hits the dragon repeatedly rather than drifting off behind him. The two characters that do most damage are, unsurprisingly, Lich and Mage (I once did over 1000 points of damage to that beast in one cast of Poison Cloud by a Lich.) A priest will do OK, but is considerably less effective. Arm other characters with wands of Poison Cloud. He seems immune to Lightning, which is the druid's major combat spell.
