Mage Inquisitor - Knight Enchanter specialization, have Energy Barrage and a stave of the type the dragon is weak to and all the basic elemental spells (Winters Grasp, Flashfire, Chain Lightning) and just weave that in-and-out of using Spirit Blade and a Decloaking Blast-enhanced Fade Blast. Bring along Vivianne, Cassandra and anyone else for your third party member (it's a throw up between Solas and a rogue, really, Dorian and Bull are dreadful at killing dragons).
Rogue Inquisitor - Tempest works well for doing huge amounts of damage, since they have a lot of damage cooldowns and their Thousand Cuts Focus ability is really good for shearing chunks out of a Dragons HP. Archery or Dual Blades works well with Tempest too, since you're just alternating the various cooldowns. Could go Artificer, but it's just easier to bring along Varric if you want one of them. Party comp of Cassandra, Vivianne and Solas.
Warrior Inquisitor - It's either Champion specialization or Reaver specialization - Templar is a powerful tree in game due to all the demons you fight, but it's quite useless against dragons. With Champion, go sword and shield/Vanguard, focus on guard generation and defensive abilities (and taunts) and just refuse to die while all your allies deal the damage for you. Good party for this is Vivianne, Varric and Solas.
As a Reaver, it's two-handed weapons and Battlemaster; focus on gear that gives high armour penetration and Guard on Hit, and balance uses of Devour with Dragon-Rage so you don't put yourself into critical-range for the Dragon's various AoE attacks. You'll do a lot of damage, but good god are Reavers squishy. Good party here is Vivianne (listen she's just really good at killing dragons), Cassandra (Blackwall is tankier, but his AI is so much worse) and whatever 3rd party member you please.