I haven't finished it yet, but I hear that's one of the toughest fights in the game. I finally did it with Shale, Wynne, and Leliana. Shale went in close to keep her occupied while Leliana and my P/C hammered her with ice arrows, and Wynne supported.
Used a similar strategy against Gaxkang the Unbound, although I added Alistair to my archers and didn't carry a mage. That one wasn't so much tough as it was grueling, and it sucks because he's virtually COMPLETELY immune to any CC spell.