Crossposted from my post in the other thread about this:
I'll lay out my strategy. It doesn't work, yet, but I got her to one bubble once in three hours of combined attempts. I'm sick of it now but maybe one of you can refine it further. This is on normal.
My party consists of a PC cipher, Eder, Grieving Mother, Durance, Aloth, and Kana. The ciphers have blunderbuses, Aloth has a bow, Kana and Durance have arquebuses. Eder has sword and shield and 10 of the level 4 endurance potions, and the skill that allows him to instantly revive when knocked out (I think it's called second wind). They're all level 12, and they all have the best gear the game has to offer. I've done everything else (all other quests (except for a bugged out one), warden bounties, etc.), as far as I can tell.
To start the battle, I move Eder up the left side and my party along the bottom. Trigger the battle by moving Eder further up. Draw aggro with Eder by attacking, heal the acid breath with a potion. Move party over to the right along the bottom, kill the xaurip priests. Move party up along the right, kill the first Agaran, then kill the second. All the while Eder tries to maintain aggro on the dragon and heals himself with potions. Sometimes he'll get charmed, you have to be extra careful then, but it usually doesn't last long.
After all the ads are dead, use the built-up cipher juice to cast disintegrate on the dragon. Disintegrate is raw damage, so it cuts through DR. You can also use your ciphers to try to stun it, but that fails about 50% of the time. The rest of the party unloads on the dragon. Freezing spells seem to work well with Aloth. Use Durance to raise downed allies who got hit with the breath when Eder lost aggro when you were walking around.
Then die, die, and die again. I hate how completely unreliable maintaining aggro is. I think one way to improve this plan is to hire a second fighter with second wind and use two tanks on the adra dragon, but even then maintaining aggro would be massively unreliable. I'd replace Kana, or maybe Grieving Mother, but in the latter case there would probably be too little dps.