I usually play a might heavy party GAAP is my favorite, but the dragon can easily be defeated with a Magic Heavy party as well.

Your Priest can cast Devine Intervention. In MM9, DI is a repeatable spell which carries no "downside" effects and can be cast an indefinite number of times in the same combat.

Just have your Magic Users cast their most powerful spells at the Dragon and have your Priest cast Paralize. Keep an eye on your stats (Mana and Hit Points) when a party member's Mana or HP get too low, cast DI. It replensihed both Mana and HO for the entire party.

I should mention here that your Priest must be GM Light Magic in order to learn DI.

Hope this helps!