I
finally beat the dragon! Took me I don't know how many attempts over 4 days. My party was level 10-11 when I started the fight. This fight on hard is ridiculous! Mega dragon + a lot of kobolds, I mean, xaurics, including a bunch of high priest healers, and for a cherry, a bunch of big hitter mind controlling Agrakas or whatever their name is that can also summon fire, water and earth elementals when they're not charming members of your party. The dragon with huge damage reductions, that when I'm lucky it's two-shotting my tank instead of using its huge area breath that insta-kills most of my party with 180+ corrosion damage. I mean, seriously?
Eventually I found a tactic that helped a lot (still, even with the tactic it took me like 7 attempts).
The tactic that worked for me was to use Kana with the level 3 damage phrase as a chant, in full armor and shield, with a potion of regen, he sneaked his way to where the xaurics were and tanked while his music slowly killed all the xaurics. While this happened, the dragon came down on the rest of the party. A lucky paralyze paralyzed the dragon, Aloth used the ray of cold, I positioned my rogue to one side while Eder tried to tank the dragon on front for a bit. I was actually surprised when the dragon went down, as all my previous attempts it took much longer to lower its health. The only thing different was that I used the cleric's level 4 damage spell, maybe it bypasses the dragons natural DR or maybe the rogue landed a bunch of crits while the dragon was paralyzed.
Good loot in the dragon's horde