Sorc Build

[StateOLuvNTrust]

You really, really, really don't need a second tree. My blizzard sorc breezed through the game solo, all leveling done on players8. I'm pretty sure there's only one case in which you absolutely have to kill a cold immune to proceed: hell ancients--and they were a joke. Up through most of A1 hell you can remove cold immunity on almost everything with good cold mastery and a lvl 2 lower resist wand--lvl 3 is also sold sometimes, although I've never found those anywhere but Lut Gholein--and I usually carry one in my backup slot for most of my leveling anyway. A few quests require you to kill monsters that always spawn cold immune in hell, but a holy freeze merc owns them if he has a good weapon (Bonehew spawned the first time for me from a chest in A2 hell when I was on lvl 60. It's popped up no less than 3,000 times since then, not to mention almost every other elite spear/polearm, but I ended using the new thresher--33% chance to cast decrepify lets him further smoketh most enemies, and usually removes phys immunity in the off chance that I run into a cold/phys immune and get a warm feeling at the though of killing him). To help out on countess runs (which, fortunately, should always be done on players1 if you're rune hunting--and of course you ARE rune hunting if you're doing countess runs) I made a +3 fire wall +2 fire mastery leaf staff, and even that can kill her minions, although my main reason for making it was to entertain myself. I got really tired of "G! G-O! G-O-M-E-R-C you got power! You got it! Goooooooo MERC!"--so I fire wall instead. Enough of that shameful business.
I stress that this was all done solo in single player. With a party to help out it would be even easier. Stick to the areas with few/no cold immunes if you like clearing whole areas, but personally I've found that clearing areas doesn't level me anywhere near as fast as targeted superunique runs. I leveled from 86 to 87 in an hour with our dear old friend pindleskin (nerfed, but not forgotten!).
But maybe this sort of thing isn't for everyone... I refused to spend on the prereqs for energy shield and so I have to cast it with a +3 lit skills +3 energy shield staff i keep in my stash, or in my backup slot for repeated short games. My style is... stingy, very stingy.