The dragon has deathblows meaning that if you have 2 or more status effects on you, you will get mega owned. Remember that Dragons have a terrify aura so that is one status effect automatically.
In terms of companions, you want these guys: Eder, Aloth, Hiravias and Durance.
Move Eder solo to the left side of the cave and give him a ring of suppress affliction. If you don't have one, you have to manually cleanse him. Eder should have potion of major recovery, potion of major endurance, potion of llengrath's displaced images and scroll of defence. You get the idea - its to make Eder's deflection/defenses as high as possible and have some heal buttons in case he eats an unlucky crit.
Position the rest of the party somewhere in the middle and south of the cave, where the bones are and close to where the spirits will spawn.
When you go hostile with the dragon the first few actions will involve cleansing everyone of negative status effects. So use your liberating exhortations, your prayers against fear, suppress afflictions etc. This is good for two reasons - you won't lose a tonne of accuracy from being terrified and Eder is less likely to get owned by deathblows.
Eder should throw up a potion of displaced images immediately, followed by a scroll of defense. The rest of your party should spend their first few actions buffing accuracy so you can reliably hit the dragon with your debuffs. The main buffs to get up are Crowns for the Faithful + Devotions for the Faithful, Champion's Boon (on your best damage dealer) and Citzel's Martial Power on your Wizard. You want to use any aoe buffs that give you +perception and lots of accuracy.
When your whole party has like 120+ accuracy, Hiravias should use Returning Storm. This is not for the dragon (which is immune to stun I think). It is to mass stun all the spirits that spawn on your party when you start attacking the dragon.
Because you have decent accuracy, you can now hit the dragon with stuff that will reduce its defenses. The main one you want to reduce is fortitude so drop Venombloom and Insect Plague on it to make it sickened and weakened. why fortitude? Because the dragon is immune to most disables like paralysis, stun and prone but it is not immune to petrification. Gaze of Adragan is a petrify spell opposed by fortitude.
Hover over the dragon to check if sickened and weakened are applied. If they are, its time to cast Gaze of Adragan. When petrified hits, it will absolutely destroy the dragon's deflection and reflex so you can get easy melee/ranged crits and any spell that targets reflex will be pretty much guaranteed to hit. You need to quickly dump all your damage, so use your big aoes and per encounter/per rest abilities like Scale Breaker and recast Gaze of Adragan.
Remember to re-suppress afflictions if they pop back up on your party. Once you land the petrify, the dragon will die very quickly. Maybe 10 to 20 seconds tops with a level 14 party. You can mostly ignore the spirit adds because they will be constantly stunned by Returning Storm. You can use a melee damage dealer or single target ranged damager dealer to clean up some of the blights as your party is buffing but once Hiravias casts Returning Storm, you can pretty much just ignore the adds and focus on the dragon.