I did it at level 11 on hard with a PC Cipher, Eder, Aloth, Durance, Grieving Mother, and Pallagina. I did use some cheese though, as I pre-applied Psychovampiric Shield and the dragon slayer debuff before the fight began. Everyone but Eder and PC removed armor, both Ciphers targeted Pallagina with Tactical Meld for extra accuracy. First spell everyone cast was a debuff, followed by a Gaze of the Adragan by Aloth. That landed, and the dragon exploded due to the x4 damage buff it gives to any attack. The adds aren't a huge problem on their own.