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

It's a pity that Necromancers aren't very strong with Chaos magic...

Yes, there's no point in using Chaos magic at all. And it doesn't really fit the Necromancer. Besides, it's not their main school of magic, so there's a risk of not finding the right spells.
In general, my idea of a good build against Orcs is: Defense, Education, Luck, Attack, Darkness. Although you can ignore Attack/Luck and take Sorcery instead. In battle, this would look like: troops in defense mode (with readiness), followed by Plague, Berserk, Dominate, etc.