Tactics against Necromancers.

By the way, here is another point when playing against Necromancers - don't let them take neutral dwellings, especially peasants and all first-level creatures - those are just free skeletons.

In one turn, you can't wipe everyone out with the Destroy Undead spell, and there might be a serious response with Dead Tide. Therefore, you must act in such a way that your troops reach the Necromancers and kill them in this first turn.