Okayyyy... ...
The following tip requires a good mouse.
I call it the Chicken Run.
Basically, all your party members are using ranged attacks, and only
one member will engage the Dragon in melee combat.
Nevertheless, cast some summons first, to absorb the Dragon's initial attack and Wing Buffet.
When the Dragon engages your melee fighter who should have been at the back until now, get your melee fighter to start running in a circle.
NOTE! Do not run toward the rest of the party.
As the Dragon chases your melee fighter pause now and then( unless you have set Autopause for spell cast) so that your mages can cast spells at the Dragon constantly.
If you have Breach, cast it when the Dragon casts Stoneskin, otherwise, nevermind and just wait for it to wear out.
When the Dragon casts, Protection from Magical Weapons, get your members to switch to normal ranged members.
Do not forget though! To keep your melee fighter running in circles.
If your melee fighter gets hit, just grin, gulp some potions and continue running.
Eventually your mages and archers will kill the Dragon.
Good luck!