Only select builds are viable in Hell, but Bowazons, Blizz/Meteor Sorcs, Werewolf or Elemental Druids, and Paladins are amongst the most popular. It is indeed much more difficult to traverse Hell by yourself because of the constant immunities and huge hitpoint amounts.
I imagine you could do pretty well with a Summon Necro up to about the end of Nightmare, but skeletons start becoming very fragile in Hell.
I found that my bone necromancer did all right in Hell. The only problem is magic immunes, which every caster will have problems with since you can't remove that immunity. The worst creature that is magic immune is the 'Undead Horadrim mage' guy. Those that shoot unholy bolt and revive undead.
Just be sure to put a couple of points in curses if you want to use a necro and you'll probably be all right.
I've always had the most fun playing with a necro, and I do have a single player bonemancer with whom I beat all three difficulties. His most important skills in the late game ended up being the bone wall and the bone prison. Being able to lock down any group of enemies while still shooting at them is a big plus, and the walls become quite strong in hell difficulty. Of course the amplify damage/corpse explosion combo is pretty important too.
I don't really recall how I got past most of the magic immunes. Exploding corpses do quite a number on them. Trick is of course getting them within range of a corpse. I believe that sometimes I had to just curse and scream and cry like a little girl until they would lose interst and wander off after a passing butterfly or something to that effect. Still, magic immunes aside, any necro build gets my thumbs up.
What about bonemancer, i know it's best for PvP but Bone Spear and Bone Spirit make huge damage + corpse explosion + bone prison(if needed for bosses)... if you have teleport you just jump around and shot spirits with 3k damage... it works for me
Use bone wall/prison to lock enemies.
Use bone spear/spirit to kill.
Use clay golem to distract enemies that fly through your walls.
Use Amp. Damage + Corpse Explosion when facing huge numbers or magic immunes.
*with no help from outside sources* in a single player game, is it possible to beat hell mode?
Having played single player for years, and beaten Hell Baal dozens of times, I can say with some confidence that yes, it's very much possible.
Quote from "Rawrior" »
What's the best build to achieve this?
IMO the best build is the one you are the most comfortable with, and have the most fun playing with.
With the summoning necro you're thinking, the build is so strong that it doesn't really matter much what you do, shoot a bow or enjoy the view, all you really need to do is cast amp and wait for things to die. If you're in a hurry, you can start casting CE once something dies.