Okay, here are some thoughts/questions from an angry young solo sorcerer. There will be *spoilers* aplenty, ye have been warned! I'm currently soloing a sorcerer, my first serious solo adventure. I just turned level 13, and I'm taking on the Master of Thralls in the planar prison. He cast hold person on me, and then he and his air elementals cut through my stoneskins and mirror images and killed me. So I reloaded and cast spell immunity enchantment and the very same thing happened again. Why doesn't that protect me from hold person?
So far, I'd have to say that this solo experience has been a blast. Some random thoughts: Some battles that were difficult with a party have been easy as can be solo. For instance, Mencar and Co. Right out of C.I. my parties are in for a pretty tough battle. With my sorcerer, I cast three spider spawns, two webs, and then sat back and watched the carnage. I took exactly 0 hit points of damage. It was one of my all time favorite BG moments. Wiped them out without breaking a sweat. Chapter 1 hadn't even ended and I knew this character had serious power.
Some battles that are easy with a party are tough solo. Slaver ship at the start of chapter 2. That was a bear solo. Stupid trolls literally tore my head off.
Favorite spells not called stoneskins or mirror image: MM, MMM, and sunfire.
Worse spell pick: After I just leveled up and got my second sixth level spell I selected chain lightning. I've used it before, never been overly impressed, but I convinced myself that I needed another offensive spell that could target more than one enemy. Well, I should have gone with my first thought. Maybe it's just me, but the damage has been pretty pathetic. Oh well, at least I didn't take infravision, lol.
Well, I'm going to go back to counting every experience point until I level up. Yes, the all important level 14 is now so close I can taste it.
Well, I'll end my lengthy rambling. If you have an answer to my question regarding spell immunity, or have suggestions on how to beat the bugger (or anything else for that matter) just give a shout. The angry young sorcerer.

you pretty much answered your own question. Spider Spawn+Web= Cannon fodder that will take up the spells. Once your summons are history, go on an all out magic assault!

If that doesn't work, I haave a question for you... What difficulty are you playing at?

Free action is your friend.

Web+Cloudkill from offscreen usually works on the master of thralls

Failing that, can you summon any elementals? they can usually take him out as well.