Lightning attacks work on him. I've just beaten him on Insane difficulty, here's how I did it:
I actually beat him by summoning a full compliment of undead, having 4 attack him and 2 attack his Lemure's and Cornugon's and keeping everyone away from him so they didn't get hit by his Sunfire's/ Meteor Swarm. My 2 tanks were using Throwing Hammers of Thunder which only did thier electicity damage, while my cleric threw Big Death at the summoned demons. My Bard was using Arc of Gold firing +5 arrows at him, but doing little or no damage, so maybe using Sparking Bolts and a big Crossbow might be better.
My Sorceror and Wizard cast Lower Resistance on him and Malison, then repeatedly Chain Lightninged him. I had cast Mord. Sword on each of them first and hasted/pumped them with Recitation, Emot. Hope, Mass Haste and so on. After each casting of Chain Lightning I had them attack a couple of times with their Mord. Swords until they were ready to cast again (for some reason you can cast and attack in the same round, but not cast twice
) Finally my wizard ran out of Chain Lightning and I Disintegrated him and he failed his save. I'm not sure if this killed him or the damage finally wiped him out.
In summary, here's my advice:
1. Summon the most powerful horde you can. Undead seem to work best as they have resistances, Demons from Gate and such wont attack him at all. Your pets will do little damage, but will keep him busy so your casters can get off their spells.
2. Use every pump spell you can on your whole party including your pets. Using Eagles Splendor on your Sorceror or Bard is also a good idea as the Charisma boost both increases the number of spells they can cast and makes them hard to save against. Storm Shell might be a good choce as it provides good Fire Resistance.
3. Mordenkainens Swords do hurt him, but not much. Not sure if Black Blade of Disaster would work from scroll, but it might.
4. Use Dismissal and Banishment spells to try and get rid of his Summonings. Use characters that are otherwise useless for this like Clerics or Bards whose spells and other attacks would otherwise be wasted.
5. He appears totally resistant to Fire and Acid damage so don't bother with them, pretty sure Poison is useless too. Cold might work I'm not sure, but Lightning definitely does. If you have a Druid or high level Ranger, cast Static Charge as soon as he appears and you have cleared out the Demons he summons (so the Lightning Bolt always targets him). Keep casting it so you end up with lots of repeating Bolts. It's a pretty long battle so you should get a few repeat hits from each casting, and even with a save it's pretty good damage - espicially with the Scion of Storms Feat. He is also immune to Insect Plague and the effects of Moonblade which I tried to use to keep him from casting. Don't Bother with Destruction either, I thought I'd at least get some damage from it but he was immune. I expect Finger of Death has the same result. You might want to try Harm though as it's not an instant kill spell so it might work.
If all else fails, try the Disintegrate thing. As I said it might be what killed him in my case. I still got his loot though (in BG2 disintegrate destroyed all the loot on the creature if it worked, not sure if this is the case in IWD2 as well), so maybe it wasn't a Disintegrate-Kill.
Hmm, I think I'll check my characters record and find out who got credit for the kill (I'm sure he will be the biggest kill), if it was my Wizard, it'll be the Disintegrate that did it.
Well that's it - undoubtedly my longest post, but then it was undoubtedly my hardest battle to date!