What skills should Necromancers take when facing Orcs on a very large map?

You might have problems with Light, as it rarely drops (2%), so it's not worth relying on. Chaos, on the other hand, is quite good; with a large amount of Necromancer magic, the damage will be significant (not as good as a Warlock with Arcane Wave, but still great). But Darkness is definitely the most useful against Orcs, and not just when playing as Necropolis.