I had a hell of a time killing it tonight. I had my cipher, hiravias, grieving mother, eder, pallegina, and durance. Here's what tactics worked without cheese:
Displacement potion on Eder, meals on everyone (meal bonuses are separate)
LOTS of full heal potions on everyone else I made 40 or so for this fight and used half of them. (10 on eder and palegina each).
Lots of potions of power / war paint (get the secret room in the blood sands),
Drake summon on Durance (since he sits around a lot this fight).
Move everyone back in the grotto, pull eder up to pull dragon. Let Eder take the full blast.
As dragon attacks, keep hiravias and durance in reserve. Try mind controlling the dragon with both your ciphers, one should get through. Use your ciphers physical attacks on the adragan, this'll probably be the last focus you get in the fight.
Let the dragon kill the adragan and any kobolds that got pulled.
Now start using hiravias. Wall of thorns right away, then use his top level flower patch ability, then insect swarm. At the same time, your ciphers should be disintegrating and stealing attributes one at a time. My ciphers never got focus back because they were both using blunderbusses, and a superb leadsplitter still can't pentrate that hide (maybe bring arquebus in the other slot).
Durance should only be resurrecting, even healing is mostly a waste of his time with the mass of potions. Save pallegina's resurrect for durance.
The tick of thorns, disintegrate and insects should kill the dragon. You can do some cheap raw damage stuff with your ciphers. You don't actually need that much damage to kill the dragon, you just need raw damage to get through it's awful armor.
Beaten in three minutes after an hour of testing. No achievement, but some awesome loot is down there.