i know its for a duo, but your distraction wont last long... the idea of a necro doing lifetap is good, but not necessary. drac's or exile's do the job. if you absolutely want a necro, i would aim for poison nova/lower resist necro.
it gets poison immunes monsters to like 80% resists, and you can manage to get 5k nova, 40k psn explosion
bone necros are useless cuz of pit lords, immuned to everything except magic.