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

Yes, the "Curse" spell is very effective at reducing rage, and "Darkness" is useful with Berserkers against Cyclops. Also, you can take "Challenge" and "Fire Traps" – they remove rage very effectively, and "Chain Lightning" is great against Orcs, and Undead can compete with Orcs. You can also take "War Machines" if your hero is Kaspar.