Guide written by Hermit_Boy on the DiabloII.Net Single Player Forums.
This is a general overview and intended for players who are not Necromancer experts.
Uses golems, skeletons, revives and a merc. Usually sits back and supports them with Curses and Poison and Bone spells. This is a very safe build. With all the minions they rarely take much damage. This is a good choice for a HC Necro. It also requires a bit more patience than other builds.
Uses direct attack spells from the Poison and Bone tree. Usually does not have many minions. They can dish out very good damage, but they often have Mana problems. +Mana and Mana Regeneration items are critical.
Uses Poison dagger or other weapons to engage monsters hand to hand. Usually has few minions and uses curses and Bone Armor. Their main problem is usually AR, and they can be slightly fragile.
There are many other builds, but most of them are all variations of these 3 main builds.
Wands and Shrunken Heads are always good items for the +skills. Most Necros seek for +skill items more than any other mod. +Mana items are also extremely useful.
Crossbows or Bows are often good if you have lot of minions so that you can deal damage and still stay out of harms way.
Items with Teleport charges are very good as they help keep you away from danger and can also be used to gather your minions to yourself.
Here is a brief overview of all the skills, how good they are, and how and when to use them.
It is almost always a good idea to invest 1 point into each Curse. They are all effective at level 1, and all of them have their uses in certain circumstances. Aside from Iron Maiden and Lower Resist, the only benefit of increased levels is to increase the duration and radius. With +skill items it is usually enough to invest only 1 skill point in the curse.
Amplify Damage This curse is one of the best. It effectively Doubles physical damage done to monsters, and in Hell it Triples physical damage done. Use this in conjunction with Corpse Explosion to increase the damage. This does not have any affect on PIs.
Dim Vision Does not affect boss monsters. Same as Hit Blinds Monster. This is nice to keep running or teleporting monsters in place. Especially effective when you have a ranged attack. It also keeps monsters from resurrecting other monsters, like unravelers and fetish shamans.
Weaken Monsters will do 33% less physical damage. This becomes obsolete when you get Decrepify.
Iron Maiden Returns damage to monsters that deal out physical damage. This is more effective at higher levels. Works especially well with Thorns aura or with an Iron Golem. Less effective in Hell.
Terror Does not affect boss monsters. Makes them run away from you. This is effective mainly as a.panic button.. If you are surrounded, cast this on the monsters to get away from them.
Confuse Does not affect boss monsters. Makes monsters attack each other. Also a decent.panic button. type skill.
Life Tap This effectively gives you and your minions 50% life leech. This enables you to leech from skeletons and other non-leechables, but does not let you leech from PIs. This is VERY good for keeping your merc alive when fighting act end bosses.
Attract Does not affect boss monsters. This is the only curse that affects only 1 monster per cast. All monsters around it will attack the cursed monster.
Decrepify This has a short duration and radius. This is one of the best curses for Meleemancers. It basically has 3 effects. 1) Targets movement speed and attack speed are halved. 2) Targets do half physical damage (this is why it is better than weaken) 3) Physical resistance is reduced by 50% (does not affect PIs)
Lower Resist Makes monsters take more damage from Elemental, Poison, and Magic attacks. Effective in conjunction with Corpse Explosion, and your Poison and Bone attack spells. The main curse for Lord of Mages build which focuses on Skeletal Mages.
Teeth Only useful in early Act I Normal. Even at high levels it is weak.
Bone Armor It is almost always a good idea to put 1 point into this for any build. This will deter 1-hit kills. Meleemancers often want to max this skill. This works in conjunction with Physical Damage Reduced items.
Poison Dagger This adds poison damage and AR to any dagger attack. It also adds AR to any melee attack no matter the weapon (a bug which will probably be fixed).
Corpse Explosion Does 50% physical and 50% fire damage. Works well in conjunction with Amp Damage or Lower Resists. This does not scale with the Players X command. Less effective in Hell due to Global PR. Also good for cleaning up corpses.
Bone Wall This can be an effective crowd control skill. Your minions will attack it tho so it is rarely used if you have minions. Life of the bone wall does scale with Difficulty.
Bone Prison Same as Bone Wall, but surrounds 1 monster.
Poison Explosion Comparable to Corpse Explosion, but usually not as good. Can be used to clear corpses or keep monsters from healing.
Poison Nova Does decent poison damage as an area effect. Use in conjunction with Lower Resist.
Bone Spear This is lower cost than Spirit. It pierces, so it is best used for crowds or lined up monsters. Use in conjunction with Lower Resist.
Bone Spirit This can be expensive. It autotargets a monster and does good damage, so it is best used for bosses and individual monsters. If you start firing at a monster, hold down the button even if the monster leaves the screen and it will still hit them. Use in conjunction with Lower Resist.
Skeleton Mastery Usually if you plan to have a number of skeletons or revives, it is good to max this. It will increase their damage and life.
Raise Skeleton They are mostly good for cannon fodder, and will rarely help you deal damage. They are cheap and good to dispose of corpses also.
Skeletal Mage They deal Elemental and Poison damage. With enough of them, high Skeleton Mastery, and Lower Resist curse, they can deal out decent damage. Basis for the Lord of Mages build.
Revives They only last for 3 minutes, so it is rarely worth more than 10-15 points including +skills. With more points it is hard to keep too many around at the same time. They are excellent meat shields, and can also deal out damage depending on the monster that is revived.
Golem Mastery Usually good to invest in if you rely on a Golem, but probably not worth maxing.
Summon Resists Helps Golems to stay alive longer and is always good to invest a few points into.
Clay Golem Usually outclassed by the later golems. If you get into a position where you need a tank, but cannot afford to keep recasting Fire Golem or Iron Golem, this is a good one to continually summon.
Blood Golem Decent Golem, but is usually only used in specialized builds. It shares life with the Necro.
Iron Golem Very good Golem. Must be created from a metal item, but then it takes the properties of that item. This can make it very strong. Has a built in Thorns that can be used in conjunction with the Iron Maiden curse.
Fire Golem Usually highest life and highest damage golem (depending on what the Iron Golem is made of). Very good for tanking.
Almost always, the best Merc for a Necromancer is an Act II merc with its auras. Especially with many minions, the best mercs are Thorns, Prayer, Defiance, or Might. The others can be good also. Remember tho, if you are summoning lots of minions, a Holy Freeze merc may not be a good idea as it can cause monsters to shatter when they die and thereby will not leave a corpse.
Diablo and Baal are usually the bosses that scare Necromancers most. Both have area attacks that can kill minions very quickly. Make sure you are stocked up on Potions, and run to town when needed.
Most other battles are fairly easy. Lister, Ancients, Council, Duriel, and other monsters who scare some of the other classes are often not bad for Necros, especially if they have a lot of minions.