My solo Sorcerer vs. Yxunomei, the Marilith, on Insane difficulty. I decided to showcase two ways to beat her: The first is without using the spell Improved Invisibility, and the second is with it. I consider this spell very powerful, but it seems a bit like cheating since Yxunomei and the priests and mages basically stand about doing nothing. Not really challenging at all. Just seems like an exploit, so I beat it the first time without the spell. Spell Immunity also makes this fight trivial since you can become immune to Dispel Magic, but I didn't bother with that one since it seems rather exploitative as well.
On Insane she has more allies (including three Mages that teleport in after the fight starts) and far more traps are placed on the floor, making the battle really tricky. The Priests are honestly the worst thing to worry about for one reason alone : Dispel Magic. With one casting all of your defenses will be stripped away instantaneously, regardless if you run away or not. I try to disrupt the casting with a few magic missiles, but the Priest keeps concentration and casts the spell anyways.
I wipe out the mages first because they will spam Feeblemind (technically instant death since it is permanent), Dire Charm (by the time you recover from the super long duration you'll probably be dead), and Magic Missile on you, which will instantly waste your Mirror Images and do heavy damage (doubled on Insane). Two or three castings and you will die.
Defense is the key to survival: As long as you have your Stoneskin or Mirror Image up, you are nigh invulnerable. Use the sightlines to keep yourself out of the archers' point of view so you can protect yourself, and once the Priests have exhausted their Dispel Magics, you can easily win the fight.
I used Protection from Fire on myself in case I needed to heal, but I had manageable health that Trollish Fortitude was more than enough. I also wanted to save enough castings of Melf's Minute Meteors in case Yxunomei didn't go down in one casting. Level three spells are probably the most important around this time of the game, and during this fight, you have to use them carefully.
