I used the term Bone Mage for the simple reason that I hate having every necromancer build end in "mancer".
I have to find my guide to repost it, but here's a synopsis:
There are several different ways of playing this build, but I'll give some basics:
For crowd control you have many different methods at your disposal. I made use of Bone Prison, as I was going to max it anyway, it did a great job not only of keeping me safe from monsters but from secondary attacks (lightning enchantment, for example), and it has a low mana cost at high levels. I don't recommend this method if you are going to use the AI curses. I used Decreptify as pretty much my only curse, so I could pump my primary attack synergies.
As Vex already said, you can use a golem for tanking (along with a merc). He is pretty durable with some +skills. I would also use AI curses with this option, but it is an option.
The other is to not use any summons and use the AI curses as your crowd control. If this is more your style, I would recommend using Bone Wall to section off enemies (like ranged attackers, lightning enchanted bosses, etc.), so they have access to each other for damage. An advantage of this method is that the monsters hurt themselves, giving an easier time for dealing with magic immunes.
For a Merc, I would go with a Rogue for this build, excepting perhaps if you are going with a Golem tank and no AI curses, in which case you might look into a defiance merc. The reason for going with a Rogue merc is that you will be using Wall/Prison to section off the enemies to prevent them from hurting you. This also means that the enemies will be sectioned off from your merc as well, making a ranged attacker much more able to deal damage.
Magic immunes can be dealt with in several different ways. One way is corpse explosion, which can be limited. Others are:
1) AI curses (Attract is a wonderful thing)
2) Merc (very easily done)
3) Iron Maiden+Bone Wall/Prison (slow)
4) Physical ranged attack on switch (I don't like this option, but others might)
5) Poison Nova items (head/wand with +Poison Nova skills) on switch (which I used to some extent)
There aren't a lot of magic immunes, so I would recommend using either the AI curses, merc, or both.
For equipment your looking for resists, +skills, faster cast, resists, increased mana recovery, and resists. Caster gear is pretty nice all the way around.
Once I get my guide back, I'll repost it. Hope that this helps.
Porius