Poison nova is the most fun IMO. You can run Pits pretty easily with half-decent gear but Enigma and Death's Web are real game-changers, opens up more farming areas and much faster runs. You can run Trang's gloves, belt and "shield" for -25% EPR set bonus which is pretty chunky in hell.
Definitely more a level-clearing char than a boss killer. Taking down a boss with necro typically involves casting Decrepify and getting your merc to do it

With some good equipment you'll be able to shred Cows and Pits, even CS is doable.