I finally beat this fight last night after trying off an on for a few weeks. Here are a few tips:
1) Don't be afraid to use items. This is the first fight in the game that I really had to spam items. Use your wands of calling to make sure that every party member has something summoned.
2) Choose a side for your ranged units. If you're willing to spam potions that restore abilities faster, choose the left side. If you're better able to deal with slow, choose the right side.
3) Make sure your tank is facing away from the rest of your party (other than your pets probably). Spam that challenge spell!
4) Weaken the top two turrets with your minions and ranged units until they are pretty low.
5) Play defensively until the poison comes out. The initial turrets are not immune to poison (the warded ones that she resummons are).
6) Wait for the poison to do it's job, all turrets should die on the same turn.
7) Blast away!
Good luck, this fight was really brutal on the hardest setting