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

Theoretically, such a scenario is possible. But firstly, you forget that the Necromancer has "Unholy Word," and you don't. Secondly, it will be quite difficult to intercept him, and it's not certain that the Shamans will go first, rather than the Necromancer simply combining the Liches into one stack, which would end your strategy.