The Necromancer is probably the hardest character to use if not one of the hardest.  He doesn’t have the offensive spells like the sorceress but is still cursed with weak starting stats and a high need for manna.  A successful Necromancer will be using his minions, spells, and curses to vanquish his foes while avoiding physical hits.  I also suggest using one handed weapons/throwing weapons, to give the necromancer access to special shields. To facilitate the defense of your character’s life, a point or two in bone armor might be a good idea just to start off with but this depends on what tactic you use. For now I’ll just say get the best armor and resistances you can get for your necromancer. I’ll talk more about defense strangely enough in the offense section.

So far I’ve taken two routes with the necromancer.  The first route is the Army General, the second one is the Commando. The first is obvious, have an army of minions at your side. The second focuses only on you and your golem.

For the Army General you'll want to gradually create a balanced army of your Golem, Skeleton warriors, Skeleton Mages, and finally Revives.  Don't fall into the trap of maxing out your numbers before hitting the masteries and resistance skill.  I usually alternate between adding a warrior and adding a point to skeleton mastery.  For Normal I usually stop at 5 to 6 with 2 mages and a beefed up golem.  Be sure you start investing in summoned resistance when it becomes available.  You WILL need this.  The hard thing is keeping your build balanced. It's good to add a couple  points (2 to 3) to summon resistance at first but afterward rotate this along with your other skills.   Finally choose 2 curses (amp damage should be one of them) and either bone spear or bone spirit.  Note: Only 1 point into the curses, use items to increase their level, add a point or two into bone spirit when your minions are holding their own.  You'll need this during boss fights but don't solely focus on it just yet.  The focus of your stat points should be Mana as primary and Vitality as secondary, but don't forget the others.

For the Commando type Necromancer, you’ll be developing more like an Amazon. You’ll want to use throwing weapons (Javelins are good) until Bone Spirit or Bone Spear become available.  You can avoid physical damage using bone armor.  Your trusty sidekick will be your golem so be sure to make him powerful through careful use of your skill points.  Finally spend a couple points in crowd control spells like poison nova and corpse explosion, your lack of numbers will make this a good idea.

Note about Curses:

Notice I'm saying nothing about attract, confuse, and terror.  Spells like these are good against normal mobs you might find in the open but they'll not work against bosses.  In addition, when you get to the higher levels these will do nothing to improve the damage you and your minions do to others. So for the most part these three are fun but not worth your well earned skill points.

For your stat points:

Army General: focus on manna and vitality

Commando: give your self a good manna pool but also give your self some strength and dexterity so you can use the good throwing weapons.

Army General: Use your merc., golem, and spells to make your first kills then raise your army from your enemies.  Your minions will make your kills for you and keep foes off of you. Support your troops with curses when they start having a hard time or use your spells to hit the problem monsters.

Commando: Use your area effect spells to weaken/kill groups at once (corpse explosion, poison nova, bone spear are good, bone spirit is good against single foes). Cast your golem to attract some of the mob and hit the ones that he doesn’t attract with your offensive spells or weapons. Use bone armor to keep physical attacks off of you and beef up your resistances.

Army General: Go in with full numbers and curse the boss with amp damage.  Distract the boss with your golem to get him turned away from your other minions while they beat the tar out of him.  When they are gone pelt the boss with your offensive spell if he's near death.  Otherwise run for reinforcements.  The first 3 bosses may end up being easy if you set up your army right.  When you go against Diablo you'll be running for minions constantly.  If you forgot summon resistance you skellies will not stand up to Mephisto let alone Diablo.  My army withstood Diablo's fire attacks for the most part.  His lightning inferno is what wiped them.  The good news is it can only attack one in one direction at any one time.  Diablo can't run while he's doing lightning inferno but he can turn (so keep running in a circle around him till he's done). 

So far in my revisit here, these are my results with this strategy on normal level (note this is normal level).  Unfortunately time isn't as plentiful as it once was otherwise I'd probably have some experience on Nightmare as well.

Commando: Use your golem as a distraction and hit him with everything you've got.  With the newer patch bone armor might be improved in nightmare/hell but I haven't tested it yet.  The biggest problem you will have is dealing with the minions as there are more of them than the boss.

Any hired minion you like is fine, as you’ll have plenty of others to take the front line, however commandos might want a front line merc. to help out the golem.

Skeleton warrior: Aggressive, stays with you, however weakest of the melee fighters (good in larger numbers, and against normal mobs)

Skeleton Mage: Close Support type, low hp but decent defense, damage vastly improves with use of skeleton mastery (good they're good at adding DPS but nothing to go crazy with.  They'll usually be the last to be attacked as they usually hang back and fire from range).

Revived Monster: The best minion besides golem.  Their role as font line or close support depends on what you raise. Raise the most powerful monsters to be your solders and watch the carnage! (caution: they will follow but are slow, they will die if you get to far from them and they always have a 3 min life span).  just keep an eye on them and they will serve you well.  Get new revives when you do the ancient's challenge.

Golem: your very trusty sidekick. Out of all your minions make sure this one can handle him self as all others require corpses to summon, all golem types are font line warriors.  The one "exception" of sorts is the iron golem who requires a metal weapon or armor piece to be summoned.

Expect to have to run for reinforcements especially with the stronger bosses. So it is advisable not to lay waist to the entire level when you decide to take out the end level boss (corpses left in an area after you leave tend to vanish).