This is assuming you don't want to use any cheese methods at all, like surrounding the dragon before you even start the fight.
Summon creatures to tank for you.
Basically acting as punching bags while your party members hack/shoot/slash/stab/impale/etc... ... the dragon
Cast as many defensive spells as possible before the fight, starting from the longest duration ones. Make sure you have either Remove or Resist Fear.
Haste is a nice spell too.
As you engage, spread out your melee combatants, summoned creatures in front and at the sides of the dragon, melee party members at the back and mages/clerics/ranged fighters just within shooting/casting range.
As summon creatures get unsummoned/killed, summon more.
Make sure they occupy the dragon at all times.
Cast these if you have them, Breach, Insect Plague, Lower Magic Resistance.
Shoot these if you have them, Arrows of Biting.
Good luck!
