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

No, that's not right. Read the manual again; it's clearly written there in Russian that you only get a few skill points for a Necromancer hero, even with level 4 Necromancy.

It seems you don't understand the game mechanics very well. Look at this. Let's say you level up a hero to level 25. Then, either your hero will be level 26, or you'll have one hero at level 25 and another at level 22 (I'm just giving an approximate example based on experience). So, check the manual and see what "units" you get. Wouldn't you want an extra hundred skeletons or an extra week of vampire lords?