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

Regarding skeleton archers – the gas sector is right. Archers won't make a difference.

And what will melee units do? They'll be wiped out with the first volley.

Also – why isn't there luck/soldier skill in the upgrade system?

Firstly, it's difficult to get luck with Necropolis; secondly, as far as I remember, vampire blindness from soldier skill doesn't depend on it; thirdly, luck can be increased with artifacts and on flags/idols/fountains.