Tactics against Necromancers.

No one is attacking them. As a Necromancer myself, I know their strengths and weaknesses well, so I'm offering advice.
You can't destroy all the undead in one turn with the "Annihilate Undead" spell, and there might be a serious counterattack with "Deadly Wave." Therefore, you need to act in such a way that your troops reach the Necromancers and eliminate them in the first turn.