PhatTrumpet said:
General PvM = slvl 17 Cold Mastery with +skills gear on
Boss-running = max
I may have simplified it a bit, but if you don't understand what Conner was going after you can just go by what I said.
I tried to post something like this yesterday but my internet crapped the bed. The idea is that, in Hell, some monsters have no resistance to cold, some have a decent resistance to cold, and the rest are immune to cold. In getting your Cold Mastery to slvl 17 you increase your effective damage against
everything, but as you go past that you only increase your effectiveness against resistant monsters (and immune monsters with broken immunities due to LR or Convic).
Bosses are resistant to cold: Andy ~ 50%, Meph ~ 75%, DClone ~ 95% (these are guesses). So if you want to get all technical about it, you'd need slvl 27 Cold Mastery to maximize your effective damage against Andariel in Hell, slvl 32 CM for Hell Meph, and slvl 36 CM for DClone.
So back to your last question: there are Pits with no cold immunes aside from the occasional Boss. I believe the most popular ones are Ancient Tunnels, Crypt, and Mausoleum. And by now, pure cold Sorcs are the Meph-runners of choice for hardcore. The choice between Blizzard and Orb seems to be more about personal preference, but I have tried both. My general impression is that Blizzard tends to be the more offensive skill and a very versatile route, while Orb is more for safety and single targets.
Blizzard's damage, stackability, and radius/range make it ideal for pit-type runs in multiplayer games and even solo Baal runs. However, investing in synergies for more damage gives you far less points for safety/support skills like TK, ES, Static, Teleport, and Warmth. It's always best to max Blizzard first, then get Cold Mastery to at least 17,
then start working on synergies, as the 5% from Mastery works on your full Blizzard damage while the 5% from synergies works only on your base Blizzard damage. Blizzard is also very effective in untwinked solo play as you can start pumping Ice Blast early on as a synergy and use it to kill (upwards of 100-120 damage as you approach lvl 24).
Orb is the "safe" route, and is just about as effective for Meph runs as Blizzard, if not more so. That's the only place it really beats out Blizzard, though. 70%+ ES with maxed TK can make for a very durable Sorc, but ES being factored in before resists requires you to have a rather large mana pool to truly make it effective, something not always available to those with average gear.
So my vote definitely goes with Blizzard.