I just completed this fight at lvl 14 Nightmare, taking 2 tanks Cassandra and Blackwell. Also using 2 mages Solas rift, storm, and spirit mage for barrier. Myself a storm, spirit mage, inquistior. Using the perk with the additional 4 potions from forces.
This fight was easy, the first attempt I failed because I played it in realtime, getting upto the add phase, but gaining an understanding of what to except.
Then I played it again using some tactical pauses and absolutely pawned.
The first thing to do is run to the boss area, get your ass to the chopper. Don't bother loittering on the small dragonlings previously to the next area where the dragon lands, otherwise they will likely drain potions, when getting hit from the boss aerial fireballs attack.
After getting up to the boss area have the tank's charge forward to engage with either front legs, making sure to avoid range fireballs prior to being in combat range, not standing in any fire which remains active on the ground. When the dragon wing bluffs attack, drawing all combat towards, make sure that any range dps have active barriers. Then those dps quickly retreat to a safe distance avoiding the dragons flame breath attack. If a ranged party member dies, don't bother reviving them until the boss flies away. The dragon at around 3/4 health will fly up into the air shooting fireballs, these are easily avoided, but make sure that any party members don't run through the flames which remain active on the ground. Go over to the right hand side under a ledge where the dragon perches on. This overcrop where your party is now directly underneath, means the dragon can't cast fireballs onto your party. During this time revive if necessary any fallen dps.
Phase 2 the same again, during the wing bluff attack, have active sheilds on any range dps then retreat, avoiding flame breath. This time when the dragon takes off at @1/2 health. It now perches on the left overcrop, avoid aerial fireballs and have your party stand underneath the left outcropping legde. In this phase the dragon will summon a few small dragonling adds @ 2-3, once these are dealt with quickly. The dragon lands.
Phase 3 during this phase the dragon ground stomps attack stuning the entire party, summoning lots of small dragonling adds @ 5. After dealing with those orignal 5 adds, more will periodically enter. Have a tank gather up the add's agro, using a rift from the inquistor talents to quickly pawn them. If lucky hitting the dragon aswell from that nuke. When the dragon files off, it flies to the north of the map landing in a new area. There will be dragonling adds attacking your party enroute to the northern hill, possibly a few on the north hill. The dragon remains there, if the party is on that hill they cannot be hit, onbly getting hit as they make the final elevated climb.
Phase 4 the dragon has landed in the north of the map, a new area with a landmark. Any small dragonling adds apart from those on the hill, don't pursue, after killing any adds on the actual northern hill. The dragon has about 10-15k hp remaining so an easy win, watching where any range stand, because this is a small enclosed area, where can get hit from various attacks.
The main things to watch out for. Making sure, the dps has active sheilds during the dragons wing bluff attack, range dps quickly moving away back to safe range's, avoiding the flame breath attack and any ground fire. Don't bother reviving fallen dps prior to the dragon flyingh away, perching onto the ledges, they will be a potion drain prior. Avoid aerial fireballs during the dragon's flight and any fire on the ground. Have an active rift for the add phase, have a tank collect the small dragonling adds which hasn't taunted prior, or they will likely get their ass toasted by dragon fire. Don't bother loittering in the main area with the small adds in the final phase, go to the north hill quickly, but pausing, regen'ing, clearing the north hill before making the final elevated climb. When making the final climb barrier the party, they will likely take a hit from a ranged dragon fireball attack, when they are assualting the final position.
An easy fight most of it was done in realtime, some tactical pausing for the add phase. My tanks never used a single potion. Solas was a potion consumer though, and a pain to keep him up, but his damage, second barrier, helped a little.