I beat the fight without anyone taking any damage on hard.
My party was eder tank/dps with sabre focus, PC as pure tank/summoner chanter, kana, eloth, grieving mother, durance.
prefight prep: had grieving mother eat two drugs to boost her accuracy
Fight start: had everyone use their figurines to summon a whole army of stuff. adra beetles, dryads, shadows, wurms etc you get the idea.
Then had everyone use guns (except cipher and wiz, they used warbow and scepter respectively) for the rest of the fight with the wall of summons in front of me, and constantly getting replenished by other figurines and lvl5 summon invocations from both my PC and kana.
Called to his bidding, ancient instruments of death are OP. It summons 3 mobs: 1 2-handed sword, 1 pike, and 1 bow. They're very damage resistent, deal good dps, and are immune to cold damage and mind control. Kinda like mordenkenain's sword from BG2 except much better. With my chanters constantly summoning these, I had a neverending flow of disposable DPS meatshields.
Had durance cast the buff that increases the time of all other buffs, then chain casted buffs non-stop. Grieving mother cast defensive mind-web, which was really key to my party avoiding all the cc cast at them and basically taking no damage at all the entire fight.
I kept using grieving mother to charm (with whispers of treason) the weaker dragon, so he fought on my side the whole time. She never missed due to the crazy amount of buffs she had on from the priest, plus the 2 drugs I gave her.
Had aloth cast wall of force and wall of multi colors on Llengrath. Then everyone worked on killing the trash, then llengrath, the weaker dragon, and finally the stronger dragon.
With the dragons constantly focused on the 2 chanter's summons, they never attacked my party and didn't do a single point of damage to any party member.
And I was lucky none of llengrath's or the mages cc's stuck on anyone. Defensive mindweb helped a lot here.