There are 50 (or more) ways to kill every monster in this game
First time I summoned wizard eye and while the dragon was busy hacked him away with my tanks and probably had my mages cast some offensive spells (do not remember which, sorry). Second time I took Vorpal Sword - two hits and he was dead. It all depends on your party, style of playing, and equipment.
Good Luck
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Mishik's Place