Out of curiousity - what level are your characters? Mine were all in the mid 30's, and we didn't really have much trouble.
I had Korgan, Sarevok, and my Kensai whirlwind her, while Viconia kept casting Greater Restoration or Mass Healing as needed.
She wasn't very tough at all - died pretty fast with the trio above beating on her.
The key seems to be as the others have said above - focus on her, ignore the minions. One important fact is that she is not subject to Time Stop, though her minions are.
Oh! I almost forgot - before I went after her I had my mages cast Chain Contingency: Abi-Dalzim's Horrid Wilting x3, it was set to go off upon seeing an enemy and targetted the nearest enemy. Took out her minions in nothing flat.

In retrospect I should have only had one mage cast it, then the other for round two.

Hrm. Next time!
I wasn't able to rest either -- but then again it wasn't really nescessary. Honestly, the guardians were more trouble than Mel was.
I hope this helps you out RT.