I've tried creating (with character editor) a longish series of characters for Diablo II's nightmare hell difficulty for solo play, and have universally, i.e. with any build I've tried, faced serious trouble keeping sorceresses alive because they couldn't handle groups of monsters with mixed immunities.
Is it hopeless, or am I missing something? The difficulty keeping a sorceress alive with the kind of build that just mows through levels on easier difficulties, compared to a much less buffed paladin or necromancer, is dramatic.
I've used the character editor to create characters with different skill trees, even "impossible" ones, but with only money for items.
Edit Originally the question asked about nightmare difficulty, which is indeed easy. I meant hell difficulty. Fortunately, most of the answers tackle hell difficulty.
Best Answer
You cannot survive even Normal difficulty with a Sorc that focuses on one skill, at least not without help (and help is rare in D2 nowadays; most have moved on to D3). By the end of Act V Normal, you will have faced at least one powerful Cold-immune (Frozenstein, and Duriel in Act II also has high Cold resistance in Normal and is immune in NM/H) and several immune monsters in the gauntlet that you face when confronting Baal. A dual-tree Sorc is typically your best bet, and you can usually get away with select skills from all three.
As another answer mentions, the MeteOrb build is the best choice for a Sorc looking to be independently effective through Hell difficulty. This build maxes Frozen Orb, Meteor, Fire/Cold Masteries, and as many synergies as possible (mostly on Meteor; FO's only synergy is rather weak). You'll also want a decent hireling as a meat shield; the most common choice is an Act II Defensive merc, because their Holy Freeze aura will slow even Cold-immune enemies. The basic tactic is to FO everything that isn't cold immune, and Meteor everything that is. The trick is to lead the target with the Meteor; most guides say to basically cast it on yourself as you retreat, and watch them walk into it (for thousands of points in damage). Large groups of monsters with no immunities can be given a one-two punch of slowing them with FO, then slamming a Meteor into the mob (hopefully centered around your hireling and not you). High-hit-points enemies like bosses are generally handled by having your hireling toe-to-toe it with them while you alternate FO and Meteor. The biggest problem with this build is that both of your primary skills have a cooldown, which throttles back the amount of pain you can inflict over time. FO also has only one synergy (besides CM), and it's not very powerful, so you'll find that FO becomes much less powerful (relatively) beginning about Act V Nightmare.
Other variants on the "fire and ice" theme include:
Pretty much all of these will give you a formidable build for Hell difficulty.
To this basic stew of fire and ice, you typically add two skills from the Lightning tree. Teleport (which requires Telekinesis) allows you to get around the map very quickly, including across chasms and moats, and is pretty much a must for boss runs (repetitively creating new games on B.Net to kill the same baddie over and over for item drops) including the popular Mephisto run, which requires exploring the largest single dungeon floor in the game (Durance of Hate Level 2, Hell difficulty). It's also useful for "kiting"; staying in front of a large, powerful enemy who has to get close to do the most damage. Static Field is also a powerful friend for enemies with high hit points and resistances; it takes a flat 25% off of the HP of any enemy in range that isn't fully Lightning-immune (think of an enemy like Diablo, with 1,000,000 HP, and imagine executing an attack that takes off 250,000 HP in one blow).
Now, don't go crazy putting points in either of these. Neither skill benefits much from maxing it; Teleport's mana cost goes down, while Static Field's radius increases. In building your character, you'll be looking for as many "+X To All Skills" items as you can get your hands on, like the Skin of the Viper Magi, an Oculus or Death's Fathom, and/or a Memory or Heart of the Oak runeword staff (Heart of the Oak, while expensive, is one of the few runewords that makes wielding a staff instead of an Orb and shield a smart choice), and in addition to buffing your primary skills, these items will give you a significant return on the one point each that you put in these Lightning skills.
I've seen Cold-Lightning combos work as well; FO/Nova is an extremely powerful crowd-control build, combining the slowing power of the Orb with a zero-cooldown omnidirectional lightning spell. Less powerful against bosses than Meteorb, because the cost per cast of Nova is too high to just be hitting one enemy for around 200 damage, and the nature and cooldown of FO makes it less useful against a single enemy. Chain Lightning, or just plain ol' Lightning, is also a popular skill to mix with a primary Fire or Cold spell, but I find these to be too temperamental; their minimum damage is 1, basically halving their average damage and making them very unpredictable, reducing your ability to "fire and forget" as you can do with most other spells.
Just remember that in Hell difficulty, virtually everything has at least one immunity, and the most common one especially in Act I is Fire. Focusing on a single skill tree is a sure-fire way to invalidate your build beyond Nightmare, but at the same time, a jack-of-all-trades build typically can't do enough damage to last long either.