Do you have summoning spells? o_O?
I have defeated the Shadow Dragon before when my average party level was only 9-10.
Basically, get summons to distract the Dragon from the front while your melee party members attack from the back.
Your archers and mages should be shooting and casting from as far as possible.
As summons die, summon more.
Oh, and as a personal tip, when your melee party member hits 50% Hp, drink a potion.
Follow these basic rules and no Dragon should prove too difficult a fight.
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If not a Sorcerer, then a Paladin.
