If you can't apply status effects it's because he manages the saving throws all the time. If anyone in your party has even a single point in Witchcraft get Drain Willpower asap. With sufficiently high intelligence the character should be able to cast DW every round and DW does not allow a save. Once you sapped his willpower he shouldn't be able to save much against effects like Blind (Headvice - Aerotheurge) or Frozen (Rain + Bitter Cold - Aerotheurge; half of the Hydrosophist spells).
His cronies can be either killed, distracted with summons, charmed or crowd controlled with spells like Boulder Bash or Chain Lightning. If you manage to keep the boss frozen he won't resummmon them even after they die.
I found this the way to go. I think "Drain Willpower" is essential in this fight.
I also wanted to add that I've noticed just checking around that more people than not have problems with this fight. Those that kill the Dragon on a few turns are the minority and they probably built the right party and there is nothing wrong with that. But I am glad this thread exists for the 85% of us who found this fight tough.
I am on my second run through of the game and so far I am romper-stomping through the fights cause I've been there before and I am already building my party with a view towards that final fight and this time around I will be stunned if I don't blow that dragon to bits within a handful of turns
