I am playing a single player meteorb sorceress and I can not complete Act 4 on hell. To access Diablo, you need to kill 3 uniques, Infector of Souls, Lord de Seis and Grand Vizier of Chaos. Now, I have faced them two times already, and both times they were immune to both fire and cold. My defensive merc from nightmare act 2 cannot deal with them either, since they heal faster than he damages them (or at least Infector of Souls does). I tried hiring the lighting merc from Act 3, with no success.
So, what do I do? Is there are chance that they spawn differently, not immune to both fire and cold?
EDIT Well, having watched this video, I realized that the way to go is just to save and exit and try again. It's not necessary that the uniques have both resistances, it's random. I got lucky on the fourth time :)