I did this with level 12 characters on hard mode, it was actually surprisingly easy because Thaos doesn't really do that much? Like other people are saying he does do some buffs initially, but the thing is, if you have two druids and a mage in your party (I was one of the druids), its just so trivial because you can basically keep them stun-locked to the point where they can't do anything.
Even if you only have the sorcerer. Just use that spell that casts oil on the floor (give Aloth some gear that boosts his intelligence, then might if possible)...you can put your fighter, the super-tank Eder, in the front and both of the giant ogres will walk towards him and your party generally. The oil will catch all of them. If it doesn't, use "maw of the void" from the druid, it knocks them all down. If that doesn't work, then use Aloth's level 5 (or is it level 6?) spell that makes them all go to sleep. Keep him casting oil. Or use your druids special petrifying claw spell (I think level 5)...there are SO many options.
Then just assault them with a barrage of nukes from your most powerful spells...its actually quite easy . I can understand the difficulty if you chose the harder path by playing with a rogue or any other character without spells...but I think if you have at least one druid and one mage it should be okay.
I will tell you what wasn't easy...the Adra dragon at the bottom of Caed Nua. JESUS. That was rough, because a single breath will knock out your entire party, not like Thaos who wastes his time casting ineffectual buffs. You basically have to use all of your spellpower to keep him prone. Near the end of the fight I forgot that dragon hunter lady gave me a spell, I used it, he seemed to die pretty quick after that .
After finishing this game I am just very impressed with the power of AOE. The priest Durance is also pretty handy...I like his falling pillars of flame.
