It never happens that one race has no chance of losing to another; the game wasn't made to be won in 5 minutes.

I really like this approach!
You can always find some kind of solution, and you need to look for ways.
At the beginning of my game, I used to complain a lot about the Necromancer's overpoweredness (how could I keep losing, while they only gain?).
Now, of course, it's funny to remember, but it's worth noting that under certain conditions, the creature conversion to undead makes Necromancers very dangerous opponents. On the other hand, the Dwarf build suggested by Mish-gan is very strong and logical.
For Necromancers, I would advise focusing on luck – soldier's luck – and recruiting Vampire Princes and Skeleton Warriors into the army. In my experience, undead with luck always looked more convincing than undead without luck.

However, with only one castle each for both Necromancers, in my opinion, it's harder to win on a long map; runes can sometimes be devastating.