1. Don't amass an army of 600 archers, because when playing on the optimal difficulty, I reached Nicholas's territory with 248 archers, having gathered all my forces and capturing everything along the way. He had only 8,000 skeletons and 800 specters. I defeated him easily. The other heroes surprised me with a small number of dragons and other minor units. Remember, a week of waiting means 18 archers and 12 druids * 2 (since you have 2 cities), while for Nicholas, it means +1000 skeletons, because with his logistics and necromancy skills, he travels his entire territory, collecting a bountiful harvest of monsters that spawn weekly on the map, plus his native cities.
2. I was slow because when the Prophet offered me 5 phoenixes, I didn't have enough space in my army and refused them. As a result, the situation was as follows: I captured all of Nicholas's cities and all the resources, but every time I fought him (he had 300 skeletons, 1000 zombies, 120 specters, 50 liches, 20 wraiths, and 30 dragons, sometimes more or less, I don't know what it depended on), he would respawn near a city, recapture it, and everything would start over. I didn't suffer any losses and brought in reinforcements that had accumulated over two months of tormenting Nicholas (if you were killed that many times, you'd be upset). Then I read this description and realized that I needed to complete the level first to get the phoenixes (since, of course, I didn't save when I refused them). At first, I was upset, but then I remembered that when I killed the vampires in my first city and checked the mage's hut, I was shown 6 locations where black dragons were based, and in the cutscene, Nicholas was shown in the background with them. My hero, after I killed the 3 nearest dragons, recites a poem. I combined these facts, tried to eliminate the remaining 3 groups of dragons, and in the next battle, Nicholas died for good.
Conclusion: 1. The faster you start the battle with Nicholas, the fewer troops he will have.
2. Nicholas can be defeated in two ways: by having phoenixes in the battle with him (alive or dead, I don't know) and by killing 6 groups of black dragons (2 in the Dungeon, 1 to the left of the first enemy mine, 3 in Nicholas's territory)