This is how I defeated Ascension -- if somebody knows an easier (or less cheesy) way, please tell me.
MASSIVE ASCENSION SPOILERS BELOW!!!
If you are allegic to cheese such as use of pre-knowledge, Abuse of Illusions, Multiple Planetars or Shapechange Mind Flayer, do not read.
Preparations before battle:
You should have Stoneskin, Improved Haste and a Chain Contingency: On Hit: Self: Prot. from Magic Weapons + two other spells cast beofre battle. Don't bother with any form of Invisibility - literally every enemy you will face except one sees through it and it is quickly dispelled in any case. Cast Project Image -] Improved Alacrity -] 5x Summon Planetar + Improved Haste on each one. Destroy the Image, cast it again and repeat. Massive cheese -- but if Mel gets to summon demons, it's only fair that I have a few celestials around. I would go so far as to claim that it is impossible to win this fight with a Solo Sorc without using multiple planetars (if you think you can do it, please tell me how).
Now, cast Simulcarum. Have your Simulcarum cast Simulcarum. Have Simulcarum1 cast Shapechange while Simulcarum2 casts True Sight. Note that Simulcarum 2 is pretty useless because he doesn't have any items (i.e. no Robe of Vecna, Amulet of Power or Boots of Speed). Summon 2 Mordies with any of the casters at hand. Haste the Simulcarums.
Position the Planetars where Sarevok, Yaga-Shura and Sendai appear. Warning: do not put them too close to where Sendai shows up - you don't want them to kill her too quickly. Position the Mordies where Abazigal, Illasera and Gromnir start off. Touch the pool (which didn't rest me for some reason - bah!) and put everyone else in the center, but with the simulcarums out of Mel's sweep. Have Simulcarum2 summon another Planetar - just so that you aren't left without any if things don't go quite as planned.
The Enemies:
You probably already know this, but I wish to highlight some of the aspects relevant to a Solo Sorc.
Illasera: She is supposedly the weakest of the Bhaalspawn - not very different from what you meet in the beginning of ToB. Don't be fooled. Her arrows both Dispel your magic and cast Miscast Magic on you - if she hits you and you don't have a Prot. from Magic Weapons (either active or Contingencied) - you are probably going to die. Her other trick is Ethereal - I must say, this works a lot better for her than it does for Imoen. She can become completely invincible for a couple of round and tends to stack this with Hardiness for 90% damage immunity thereafter. It might expire during Time Stop though... Illasera is immune to Intellect drain.
Gromnir: Nothing special here - he slows whatever he hits, but otherwise is not remarkable in any way. In fact, he is not even immune to Int drain - and has less than 10 Int.
Abazigal: He goes through anything at enormous speeds. A planetar dies in under a round if he is using Improved Haste. To make things even more fun, he starts off with a slew of buffs that include (but are not limited to) Prot. from Fire, Magic Energy, Magic Weapons, SI: Abjuration and Improved Haste. Immune to Int drain (I think - can't quite recall a time when I cast Time Stop and he wasn't protected from Magic Weapons).
Yaga-Shura: Immune to Int drain, and 99% resistant to every form of damage except Magic Cold. This is supposed to go down during the fight, but I have never done more than 1 damage to him at a time (see below).
Sendai: Don't let her cast Time Stop. On the other hand, don't let her die first - she is relatively easy to kill with planetars.
Sarevok: The most pitiful enemy I have ever faced. He literally cannot hurt you or anything else that is on the field. Nor is he immune to Int drain. Kill him at your leisure.
Melissan: Immune to Time Stop, Magic Damage and +3 weapons. Not immune to fire. Has an annoying tendency to gate in assorted vermin. More later.
Assorted Vermin: This should be divided into two categories: Fallen Solars and everything else. Everything else falls fairly quickly to planetars. Fallen Solars are immune to +3 weapons and I have almost no way to hurt them - to kill one would involve a Time Stop followed by Lower Resistance and spells of mass destruction, but I cannot afford this when Mel is on the field. Fortunately, she doesn't seem to summon them when most of her servants are dead.
The battle:
Mel shows up, summons the Five (minus Balthazar who is on my side, Gromnir is there in his place) and then calls Sarevok. Were I not soloing, I could have avoided this by taking him into the party before and talking some sense into him - but as it is, he is my enemy regardless of what I say. As soon as Mel leaves and you have control, Have Simulcarum1 begin casting Time Stop. You yourself cast Project Image who then proceeds to use Focus. Hopefully, the Time Stop should go off before anything happens.
The purpose of this Time Stop is to eliminate the right hand side (Illasera, Gromnir and Abazigal). Have both illusions cast Improved Alacrity. Now comes the tricky part: while the Image is throwing Horrid Wiltings in the general direction of Illasera and Abazigal, have the Image run up to Abazigal and cast 4x Pierce Magic on him. Then, stand a little further and cast Breach. Summon a Wizard Eye to see Illasera better. If she is Ethereal, never mind, focus on Abazigal. When you feel your Time Stop is about to end, cast Time Stop again (with the Simulcarum of course) and Improved Alacrity again. Now, give Abazigal everything you've got: while the Image runs off to cast Imprisonment on Yaga-Shura (he may lose Focus eventually, cast it off a scroll to be sure you'll get it off), the Simulcarum unloads 6x Scull Trap and 3x Sunfire between Illasera and Abazigal. Illasera should come out of Ethereal - once she does, run up to her and cast 3x Lower Resistance. Then, cast 6x Melf's Acid Arrows and 6x Magic Missiles on Abazigal (this should all be during the same Improved Alacrity as these spells have 0 casting time with the Robe and Amulet). When you are done, Shapechange into Mind Flayer and kill Gromnir. NOTE: this will end your Time Stop prematurely - after a second or two, Mel shows up and the Time Stop simply ends - this is why it's important not to kill Sendai too soon.
When the Time Stop ends, ideally Illasera and Abzigal should die - this did not happen with me, but tossing one last Scull Trap (by the Image) between them miraculously killed them both (the straw that broke the camel's back...). This was also good because it knocked Mel down due to feedback. Sendai died very shortly afterwards - with her Prot from Magic Weapons expired, the planetars got her to Near Death, she teleported away but one kick from Balthazar put her out of her misery. Sarevok was also quickly dispatched by the now-unoccupied planetars (rest in peace). At this point, Melissan gives her "Nearly mortal" speech and it looks like you have won, right?
Wrong. First, she casts Freedom releasing Yaga-Shura and I could not interrupt her. I Imprisoned him again - she cast Freedom again. She had cast Gate the round before so my planetars were distracted. The Simulcarums had long since expired and the project image was killed when Mel damaged me somehow, so I cast a new Simulcarum and summoned some more Planetars (all but the lonely last one were gone). The planetars took care of the assorted vermin -- thank goodness she didn't summon fallen solars. Yaga-Shura refuse to die -- usually his resistances go down, but either this is because I was playing on Insane or this is a bug, they stayed put. The planetars whacked at him and my Illusions used Improved Alacrity -] 6x Magic Missile (this does exactly 30 damage to him) until he finally died. Balthazar, some of the other planetars and an Image I conjured up kept Mel busy. She kept summoning stuff, but each time it was a little weaker than before. Finally, it was just Mel against everyone else - but she is quite difficult to hurt. I used the planetar's Insect Plague, Pierce Shield off of an Illusion followed by Flamestrike or the like until she finally got down to Near Death. This took quite a long time - she managed to cast Time Stop (through the Insect Plague!) and killed off some Planetars. My Illusions expired and I cast new ones - finally, when she was near death I had a Simulcarum cast Improved Alacrity -] 5x Dragon's Breath, obliterating her last HP.