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

In my opinion, it's better to keep the troops together and crush everyone with one iron fist, even if it takes longer. Again, I don't understand why experience should be divided among everyone when you can level up one hero really well. And the skill increases by only a few points from heroes, so the bonus is minimal. It's the Bone Prison spell and the artifact with the skill that have a significant impact, not Necromancy itself. Check it yourself and read it in the manual.