I hard saved before fighting the dragon, and managed to kill it on hard.
I can record a video later this weekend. What I did was carry some scrolls of Resurrection on Kana, since he and Eder were the only two who (at full hp) could reliably live through the flame breath. I sent Eder up, turned the dragon away from the party, and durance buff and GM remove his armor and buff the fighter with the pain whatever spell. Aloth kept the dragon petrified (lvl 6 spell) throughout the fight. Focused on him, killed him, make sure to have durance protect against mind control (epsecially himself).