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

Theoretically, such a scenario is possible. But first, you forget that the Necromancer has "Unholy Word," and you don't. Second, it will be quite difficult to intercept him, and it's not a given that the shamans will go first, and the Necromancer can easily combine the liches into one stack, and your strategy will fail.

Mortulus, it all depends on the skill of the player and who has more experience and is more cunning; they will win. You are talking about tactics for the Necromancer, but I, for example, know tactics for the Necromancer and even against the Darkness, where Chaos or summoning will be even more effective than Darkness (here, neither goblins nor shamans, nor resistance to magic are considered) :). As for interception, try to intercept Argath:D, who runs across half the map in one turn; the Necromancer has nothing to counter, and what's the point of the Orc even fearing the Necromancer, even a weak one with 10 liches and Black Dragon Poison, if it's the late game and we have legions of goblin shamans, and the chieftains will support them. I have never seen an enemy hero go before the chieftains!