Tactics against Necromancers.

Well, getting Utopia on the first map is a pipe dream. This is only possible if you get very lucky with the mines. But getting Utopia on the second map with Galtran (on the Fortress) is much more realistic. In short, the essence of the tactic is to capture Angels in mines or Wyverns from trees, and then distribute them to different Necromancers. By leveling up several Necromancers, 3-4 thousand skeletons by the 2nd month is not a limit. We capture secondary castles and transform creatures of level 1-2 into skeletons using the transformer. We find peasants and send them to the transformer as well! You can also send level 3 creatures from other castles there.
We try not to lose the skeletons, and then we combine them all for the final battle. If the hero is Galtran, the skeletons will have very high stats. This is especially unpleasant if there are rings of life and potions of life.

If you want to know more, search for information on this forum.

And here is a link to saves from very strong players in Heroes 3. There are no Necromancers there, but it is still worth looking at to understand the tactics of good players.
http://randommaps.siteburg.com/indhallbestnd.htm