
Don't know what else I can be doing. First of all, it's a three phase fight with the main baddie getting stronger each time, with secondary quite tough fights in-between, and no ability to rest.
My -20 AC, mega-weapon dual-wielding main character with 150hp and resistances to just about everything gets three-shotted in the first fight, while the rest of my party gets thrown about like ragdolls. After half a dozen attempts, the only way I could beat that first phase was to cast all my biggest spells and summon all my big dudes including a juggernaut golem and planetar. Which means by the second - tougher - fight, my guys are already half-dead and I've got no more good spells or summons.
I know I've beaten it in the past, but no idea how. So I think I'm going to call it there.
Icewind Dale 2 next!