Poisonmancers are more than workable. Like Tav said, you have 40+ skill points to work with, which means you can make a rather viable back-up attack. You can max Corpse Explosion, build yourself a small skeleton army, use one of the golems, use some revives, you actually have quite a few options. There are only a few monsters that I have seen in the game that have a double immune that work against you (besides the random mods of uniques). All in act 5, there are wraith types that are poison/physical immune. And then there are Skeleton Mages who are poison/fire immune. The wraiths are far more deadly than the mages, and the wraiths appear in the narrow passages trying to rescue Anya. I'd say only having to worry about the immunity of one monster is Hell is a very good build.