The most efficient build for the Necromancer (maybe even for the entire game), is the "Fishymancer". It allows you to go through Hell and back without much/basic gear, perfect for first characters with the intent of magic finding better items for future characters. You could even go through the game naked.
In summary, the skill build:
- Raise Skeleton (20+)
- Skeletal Mastery (20+)
- Corpse Explosion (20+)
- Amplify Damage (1+)
- Clay Golem (1+)
- Deceprify (1+)
(Clay Golem + Deceprify to help with bosses)
Merc: Act II with Might aura
Ideal items: +resistance, +skills, +mana per kill, magic find, +runspeed, +life/vitality
Those are all the skills you need. That leaves for a lot of skill points to put in any skill you want, and it wouldn't really matter since the above skills synergize perfectly. And Corpse Explosion doesn't have a lot of immune monsters.