With ascension mod installed, it is possible to get "Immunity to Time Stop" as a special ability from the final challenge. To late to be a whole lot of use. Other than that theres no legitamate way to do it.

I have no idea whats happening with Cereks character, but my lvl 40 Monk (~115% MR) was always affected by Time Stop.

Ah, thanks, Andrewas. I just put paid to Abizagail, and was wondering whether it was worth it worth it to finish it. I guess I will, now.

I had considered creating an item that conferred immunity to Time Stop, but junked the idea because of the scripting problems it would create: The enemy Liches and other Time-Stoppers simply aren't scripted to handle a PC who's immune to Time Stop. Let's take a basic fight of Inquisitor vs. Lich:
Round 1: Lich sees enemy--Chain contingency of Stoneskin, Spell Trap, Protection from Magic Weapons. Lich begins casting Time Stop, Inquisitor begins casting Dispel Magic.
Round 2: Lich's protections are dispelled. Lich finishes casting Time Stop. Lich does not realize that his spell didn't do squat and starts casting Meteor Swarm. Inquisitor goes *whack, whack, whack, whack*. Lich dies, feeling very confused.

Such an immunity would be really overpowered--unless the enemies were scripted to notice that an enemy was immune to Time Stop. Unfortunately, that would take a serious BUTTLOAD of new scripts, so I considered it wiser to avoid the whole situation altogether.

Hmmm... The question is, does the Lich really knows (from the scipt's point of view) that time is stopped? It's a mere speculation, by i assume the basic Lich script is more like:

(1) If CC <> 1, release Chain Contingency, CC = 1. Else
(2) If TS <> 1, cast Time Stop, TS = 1. Else
(3) If MS <> 1, cast Meteor Swarm, MS = 1. Else
...
...
(x) After Y time units passed, reset variables.
...
...

So whether you make the item or not is irrelevant - the Lich is dumb either way, it's the fact Time Stop isolates you with the target (assuming you are capable of taking him down) is what makes it so grossly unbalancing.

Weimer's Smarter Mages change things a little by having much better targeting.