Just a layout of my current necromancer. Summoner build and just broke Act 4 Nightmare. He might be a bit slow, but there isn't a lot that can stop him. I support my skeletons with nearly a full repertoire of curses so I won't have to rely on weapons and heavy armor. Later on I'll drop a few points in Dim Vision to pacify those nasty wisps you can discover in Act 5.
My hireling is very important. He's the hireling who uses the Might Aura, because it best supports my minions. Eventually I'll probably want Duress and probably some elite ethereal pokey stick that I can drop Strength into. It might also be nice to have an armor that grants immunity to freezing as a backup, because the only helmet that grants immunity to freezing has a Dim Vision proc. Speaking of helmets, Tal Rasha's Horadric Crest is the usual helmet, although there are plenty others that are suitable.
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It's a nice armor, but that won't help you much by act 3 Nightmare. Sheer defense isn't going to...