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I completed it on hard difficulty in 3 months. There were practically no losses. First, I cleared the surface completely, then the dungeons. I didn't like the Blood Superpower at all; general resurrection is much better. I mainly focused on strengthening skeletons; they had the best damage, especially with a bow and direct fire, but of course, liches were the main force. Battles against neutrals felt like target practice, and the remaining enemies were finished off by ghouls and wraiths. I didn't listen to the advice to not leave the dungeons for later; I didn't want to bother with it. I took one fort at the top, then one on the left, gradually expanding my territory, and then the city at the top. I prioritized dwellings and growth as the primary task (after resources, because there weren't an abundance of resources throughout the game).
Demon heroes didn't cause much trouble; they couldn't hold a fort for more than a day, and in general, the city's portals are very helpful for controlling your lands. I didn't rebuild the forts (why would I?), only the dwellings. And I didn't build an improved portal in my city; there was already one built in the demon city above, and that was enough.
The map is large, the game is long, and for a high-level necromancer, that's always a bonus. Difficulties arose with the 120-unit lords, and even more so with the spiderlings. 3400 liches was the largest number I've seen, but the worst was 1500 fanatics together with 700 fiends in a castle with a hero... But I managed to do it without losses, and only because the enemy didn't use powerful spells. His maximum was general weakening and various explosions (what is a 3% damage reduction or 200 damage to 3-4 units?! It's nothing!).
I enjoyed the battle with Uriel; I was relaxed. I didn't protect my troops; I killed the angel with liches (10,000-12,000 damage), standing in the corner and guarded by vampires, whom I sent to defend (neither angels nor demons are scary to them in defense). Then I finished off his guardians, who were stuck to the worm.
The campaign never felt impossible.