Defeating the dragon should be quite easy if your character is competent with a bow. Remember to use any bonus to ranged weapons items or spells if you need them. In melee combat I think the trick is to attack when the dragon is rearing back to breathe fire otherwise he can hit every time. The battle should certainly be easier with the trap disarmed. When I finished the game I couldn't find the trap and was surrounded by summoned creatures.
Fortunately I had stockpiled firepowder kegs from Weng Choi. These MIGHT have an impact on the dragon. These always seemed to work for me
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