LOL. What epic feats HAVE you been taking? Epic spike traps = awesome, instant win. (They don't require Set Traps skill checks either.) GWW = only thing you need to beat every encounter. Engage when they're standing right next to Mel, have them wield the biggest single weapon possible (with 10 attacks per round whether you're single or dual wielding, no point in using a crappy offhand weapon) and whale away. PFMW = Protection from Magic Weapons. Temporary caster protection. What they should *always* have up is Stoneskin and Mirror Image. (And, in this case, Negative Plane Protection.)
Honestly, I've forgotten how to do this battle because (1) it was really easy, and (2) I only play Ascension now. In Ascension she has some ridiculous buffs you need to dispel every time, but I don't think this applies in the regular one (and besides you have Carsomyr). To be sure, zap her with Ruby Ray of Reversal and Breach right after another (stagger the casters) right after she appears. Breach blows away the Stoneskins she has up. There are wands of this too.
You should probably replay the whole finale with these strategies.