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

How will machines help the Necromancer? How much will the Necromancer's magic weaken if an Orc levels up the "Dark Dampening" skill? Explain what "You don't need to go for the ultimate" means; I didn't understand it.

Added 7 minutes later
Can you suggest a development strategy for Dungeon against Sylvan in the "Question" thread? In the Dungeon section.