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

Even with the darkness resistance debuff, the spells of this school are still the strongest weapon against orcs, especially at level 30. Of course, you can try to develop light magic: again, by level 30, I don't think it will be difficult to find the necessary spells, either in your main guild or in secondary castles/guilds on the map/spell libraries. But the only question is: are you sure that light magic will appear? And if not? And if it appears too late, around level 22? What, will you just run around without magic until then?

but I don't see any special advantage for necromancers in this battle.

The advantage is that, with at least one secondary castle, you will be able to gather such large armies by level 30 that nothing will save the orc. And another question: are you going to level up in a circular pattern? This set of skills looks... hmm.