This thing wasn't planned to be beatable.
Maybe some rude tactic could be more effective, I don't know.
My "elegant" (from my PoV....

) and elaborate tactics don't work, 'cause the thing is completely cheesy and bugged....
I have only three chars, so everyone has to give the hell out of him/her....
Planned carefully a complicated fight with Simulacra and Project Images, all crossing their Time Stop with time stop immunity, to be able to do incredible things in a single round of "real" time.
Well, two things may happen, from my experience:
1)You do and incredible and perfect work, such as to deliver an obscene damage to the five.
Possibly killing two or three of them before the battle even starts.
At this point the "near death" bug shows up and you can only
smash the table with your head.
2)They DO die when they should. Worse than ever, Melissan appearence is scripted at the first death, and BREAKS without reason ALL your active Time Stops, breaks spell casting and the like, breaks ALL that you have slowly and painfully built.
The following so, is simply impossible.
ONCE all went well.
Don't know how, yet.
SO well that at a certain point all the five were dead, I had yet a FULLY buffed simmie, Sarevok and Immie still alive!
And I even got the "rest option" from wish!!!!!!

Only Melissan remained, and I was stronger than in the beginning!
I started pounding her, I had TWO kensai/mages in FULL-FURY and protected mode, the "real" one even shapechanged in golem to do maximum damage!
Sarevok too got his new clone, whirlwinds and all, Immie was taking down her protections constantly...
I thought this over...
At (I assume) 50% HPs or so Melissan (scripted event) teleports away, summons immediatly an HORDE of uberpowerful demons and in the blink of an eye I had Imoen and Sarevok torn to pieces, my main char alive, but as you imagine little could be done... you can't do with one char, she raises constantly Divine Spell Trap, while you take the time to remove just ONE of them, Melissan AND the demons (casters) remove all of YOUR protections and you're dead meat.
I know, there's a way maybe.
But if an experienced player (though NOT the BEST player, of course) could not do it in 5 days of incessant reloads and plannings, something is wrong.
Maybe you have to do with a full party, or make a more "summon oriented" battle (which I don't like).
BUT, I don't like the fact that you can't use creative ways to win, and that they all are basically immune to everything, even to stats
drain!!!!!
They die only when they want to, Melissan has all scripted and unavoidable moves...
Hmpf.
I do not suggest to install this Mod. Really frustrating.
(And it's a real shame, as the story was REALLY improved by it...)
But, if you want to try...good luck....