Levels 1-5: It doesn't really matter at this point. Hell, at this level you could walk up and attack the enemies yourself, and it'd be a good strategy until level 12 or so.
Levels 6-17: You'll be summoning a lot of Clay Golems, I'll tell you that. Strategy still isn't too important here, but start using Skeleton Mages as soon as you can. Normal Skeletons will lose their usefulness shortly after getting those. They may be good for taking a few hits to make your Iron Maiden go in to action, but they take so few hits that the Mana cost is a bit much at this level. Mages that fire cold are usually my favorite, because they slow the enemies, but in this area that might slow things down, since Iron Maiden is based on your minions getting hit. Poison takes time to have a good effect, and lots of enemies have resistance's to fire. The fourth kind of Skeleton Mage is lightning, but their missiles can be easily confused with those of a lightning enchanted unique monster, so they can cause a bit of jumpiness, especially in Hardcore mode. Or you could just take what you get for Mages. :) Use Corpse Explosion to kill monsters if you're in a one person game, and there are a lot of monsters right next to the corpse. Use Bone Walls to shield yourself from troublesome lightning enchanted monsters, or to hold of groups of monsters if in a narrow hallway, or a door. Use Dim Vision to hold off archers, or swarms of monsters. This can be quite useful in areas such as the sewers in Act 2, where one group of monsters can lead right to the next. Start putting more points in to Iron Maiden, Skeleton Mastery, and Golem mastery, or Bone Armor if you need it.
Levels 18-23: Now that you have a Blood Golem, your strategy should change a bit. Use the combination of Blood Golem and Iron Maiden for both your main attack, and to keep yourself constantly healed (see the descriptions of Blood Golem and Iron Maiden in the skills section for more info), and raise Skeleton Mages to speed up the process. Firing Bone Spears will also help to speed up the process a great deal in annoying areas like the Maggot Lair. You'll get Confuse and, if you decided to take it, Life Tap at this level. Confuse isn't a very effective curse. It's basically a wide area ineffective version of Attract. I never use it. Life Tap will be useful if you're teamed with other melee classes who have little or no life stealing. Continue putting points in to Iron Maiden, Skeleton Mastery, and Golem Mastery (if you haven't already put in 5, or as much as you want).
Levels 24-29: This is where you'll get your Iron Golem. Most of the time continue to use your Blood Golem, but if you find a nice weapon that isn't worth keeping, trading or selling, make an Iron Golem out of it. When made out of a nice weapon they can be quite effective, even in nightmare or sometimes hell. They heal quickly, so waiting a short while between battles can be good for your Golem, or, if you have it, you may wish to curse the enemies with Life Tap. Going back to town is always the best way to get a non-Blood Golem healed though. Use Bone Prison if you want to trap an enemy with your Golem to keep them from running away (useful against unique archers and unique blowdart Flayers). But remember that your ranged minions cannot shoot over bone walls or prisons, and all of your minions will attack the barriers, until they are knocked down. Now that you have attract, you can try the strategy I mentioned in the skills section where you curse a group with Iron Maiden, and curse one or two with Attract (depending on the group size), and watch the fun! This is a good way to shrink large groups of enemies (especially the high-damage zombie class) when your minions are not readily available, for speeding things up, or if there's just a lot of enemies. Not to mention it's great fun to see a dozen Flayers turn on a Flayer Shaman. :) Continue putting points in to Iron Maiden, Skeleton Mastery, and Golem Mastery (if you haven't already put in 5, or as much as you want).
Levels 30 and up: At level 30 you'll get your Fire Golem and Revives (and if you took them you'll also get Bone Spirit and Lower Resist). Fire Golems have the great ability of absorbing fire damage and turning it in to health. This is quite noticeable when fighting Shaman in Act 3 with their powerful Inferno spell, and you see your Golems health go up. Revives will also be quite useful, and will make up the bulk of your small army. Use your Golem (Blood, Iron, or Fire) with Iron Maiden to start things off, and get your Revives going.
Fighting Andariel: Use your Iron or Fire Golem and Revives with Iron Maiden (or Clay Golem, Skeletons, and Skeleton Mages, if you're fighting her in Normal Difficulty). You may need to re-summon your Golem and Revives (or Skeletons). Try to team with a Sorceress who knows the value of using Static Field on bosses. As always, focus on getting some poison resist or poison length reduction before fighting her. The set Leather Gloves (Death's Hand) are quite useful for this, in any difficulty. Never use a Blood Golem, or you will regret it.
Fighting Duriel: Necromancers have the easiest time with Duriel. They pick on him cause there's nothing else to do. :) Just whip out your Blood Golem, curse him with Iron Maiden, and what him tear himself to pieces. Be careful not to let Iron Maiden wear out, or you'll regret it when you realize how much damage he actually does (you'll see your Golems and your own health drop quite fast). Revives are not needed, but they help a little. As always, try to team with a Sorceress with Static Field, although with Duriel it's just to speed things up, since there's no great danger. Other classes are quick to team with Necromancers when facing Duriel, since he's actually very dangerous to other classes. Only use a Blood Golem, since other Golems will die from Duriel's onslaught.
Fighting Mephisto: He's a tougher one. Try not to use many corpses in level level 3 of the Durance of Hate, and summon as many revives as you can as soon as you are about to face him. He has a very powerful attack which I call the Lightning Orb of Doom which will take your Fire Golem apart quick. Teaming with other classes is very helpful, especially the whole Sorceress with Static Field thing. Try not to be standing in the direction he's facing, even when fighting minions and party members, since he has attacks that go through minions, and will still hit you. Also it's never fun to be hit by a Lightning Orb of Doom that someone else dodged. Always remember to keep moving, and never use a Blood Golem.
Fighting Hephasto: Bah, Hephasto's a wuss. The first time I crossed paths with him I didn't realize I was fighting him until a moment before he died. Necromancers kill him regularly in Act 4 Hell games, and other classes (especially Barbarians) get killed by him regularly, or just avoid him. Killing Hephasto in Act 4 Hell games is a good way to get experience, since no one but the Necromancer will kill him on a regular basis. Use the same Blood Golem Strategy as you did with Duriel.
Fighting Lord De Seis: Pain... Hurting... Hide behind your minions and bone walls. Iron Maiden isn't terribly effective, and neither is Amplify Damage, but you may want to use it if teamed with fighting classes. Teaming with a Bowazon would be the most useful here, partly because they can fight well from a distance, and because Amazons get the Slow Missiles spell, which makes De Seis's projectiles slow enough to dodge. You cannot make revives out of his minions, and the Attract curse won't work on them. There's a reason his name sounds like "deceased".
Fighting Diablo: I suggest fighting the Infector of Souls last before opening the fifth seal, since his minions, being extra fast, make good Revives. Your Revives will likely be gone before too long, most likely from the Red Fiery Death rather than from the Revive time limit. Depending on your luck, and the level of your Golem Mastery your Fire Golem with either die instantly from the Red Fiery Death, or (in some odd cases) not seem to take any damage. Most of the time he either dies instantly, or is very seriously wounded. If you're fighting in a party, just summon more Fire Golems, and use Iron Maiden, or if it seems more effective or your party members request it, Amplify Damage. With Golem Mastery and plus to skills a Clay Golem can take a surprising amount of punishment for the minimal Mana cost, and is still just as distracting as a Fire or Iron Golem. When going solo against Diablo (which can be quite difficult) head strait up from where Diablo spawns, and hide around the small outcroppings at the corner, and summon Golems without fear of being hit, only popping out to re-curse, or when necessary, switch the side of the corner you're taking cover behind. Here's a screenshot: