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I don't know "what's so difficult about Necromancers" for you, and in general, how can they be difficult?))
It's one of the easiest and most effective factions in the game (especially on small, medium, and even large maps – on huge maps, you need to carefully plan your equipment and tailor your skills to a specific opponent).
Try the ever-popular Kaspar (who is picked by almost everyone in the юбилей edition – an ultra-aggressive rusher; only Hafiz is better, but that's a whole different story). Seriously, you'll have a great time playing as him. You build a tent and occasionally help with a hero. I won't tell you what to upgrade for the hero because there's no absolute best choice; everyone picks what they like (even attack with speed, I've seen a few experimenters do that), so it's a matter of taste. The main thing is that you won't lose troops, and the start is simply amazing.
Or another good hero (though less popular, but still one of the best for Necromancers) is Nadir – here, you'll definitely have to upgrade two skill trees, but the start is also relatively good, and playing him later on is a pleasure.
Dark Energy can be a problem, but try to spend it on useful units. At the beginning, skeletons/ghosts, then vampires – prioritize them or adapt to the situation. But this is also a purely personal preference.
The fact is that Necromancers level up much faster and easier than many other factions (because everything depends much more on the probability of getting the desired skill, i.e., luck, which is a priority in this game)