[SixOfSpades]

I've only fought him once, but happily I was able to nail him and his buddies on the first try. My steps to victory:
1) Realize that Weimer was probably going to pull something of this nature.
2) Have the proper party balance. I beat him using a tactic that Nalia, Imoen, and Jan can do, but Edwin and Kelsey cannot.
3) Have at least 3 Level 9 slots at your disposal (or the equivalent scrolls, if you so choose).
4) Have a thorough understanding of the fact that Time Stop does not actually stop time.

Leslie, you actually already know the answer to Ira'karoudnideg's bizarre spell immunities. It's in your Bag of Holding as we speak.