Tactics for playing against a Necromancer.

I don't know, I think I would lose against Necropolis with Orcs (against an equally strong opponent), even with Anti-Magic. And does Anti-Magic really help that much?
Berserk will still work, as will Subdue. Let the spells cost more; there's Spiritual Link. Witchcraft will decrease slightly – it's not a big deal; it only affects the duration of the spell, not its effect. As a result, the Orcs will attack themselves, and there will be no removal of charms or Anti-Magic. The Necropolis will have a larger army, so the battle will be long. This means the Orcs will attack themselves for a long time.