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

Two castles for the Orcs is quite strong, I would say. In that case, the Necromancer should have at least 4, otherwise defeat is guaranteed. Although... with cunning, you can win with fewer. I won't go into details about this, as I don't want to create unnecessary chatter, but keep in mind that you probably just encountered an inexperienced opponent.