The longer you play, the stronger the enemy's troop growth becomes. But a blitzkrieg is impossible. For example, right now, it's the 2nd month, I just took that "Orange" town and sent Talanar there. And someone needs to go get artifacts in the dungeon after receiving that quest in the hut; it doesn't have to be Faydaen who gets them, but he's the one who receives the quest and is the one who turns it in. So, maybe send Talanar with undead creatures.
By the way, in this mission, Fedya got Resurrection, even though I built guild halls to the max everywhere.
P.S. Slightly off-topic: about 4-5 months, I played the "Battle Cry of Freedom" scenario in Orcs very slowly and deliberately twice, and both times the result didn't make it into the top 10; I played for 6-7 months, and the hero reached level 26.

I didn't get to the point where 666 dragons would join Nicholas every day, even though I encountered 5,000 skeletons and other creatures in battle against the enemy. I wasn't worried at all; a well-developed Faydaen with Darkness and Light magic is simply devastating, thanks to Vadimh for the advice to develop Darkness for Faydaen; it helped a lot. And as I said before, the battle with Nicholas was a joke; it didn't even start (archers shot everyone down before the battle even began). So, I killed him on the first try, but it wasn't very fun. In general, the battle against Nicholas and Markel on the hero disappointed me a lot; if I play it again next time, I'll either play on a different difficulty or change the hero's build from the recommended one to a non-standard one.

P.S. By the way, it's actually better to capture the "Orange" town as early as possible; I did it on the 2nd week, as advised in the walkthrough on Hugo's site.

Added 6 minutes ago

You played by killing dragons, and I played by collecting artifacts :)
Well, I'm not a professional in Heroes 5, and in my opinion, in the early stages, there's simply nothing to even break through the garrison with vampires and other creatures around the "Narkograd" castle or whatever it's called. That is, even if you break through, what will you use to attack the "Orange" town and beyond?
Now I've got phoenixes, I'm going for reinforcements, but Nicholas isn't visible this time. During the first playthrough, he chased me through the dungeon and attacked on his own. And Faydaen with phoenixes now shines like a spotlight.

What he suggested can be done using the "single units" tactic. You cover the archers with them, and that's it; the main thing is to have at least 20-30 archers, then the dragons shoot well. The tactic is quite tricky, but you can get the hang of it; it takes practice. And of course, it will only work 100% if Faydaen is well-developed, that is, he has Elven Accuracy, Archery, Expert Attack, Expert Light Magic, and Darkness is also desirable). I think then it is quite possible to defeat the dragons right away.
Or you can defeat them with Talanar; he has a super ability, and Luck always works).

However, I didn't do that myself; I captured the "Orange" town on the 2nd week, then went into the dungeon, cleared everything there, got the phoenixes, came to Nicholas's territory, he got nervous and holed up in the castle, and I defeated his additional heroes with the main force. By the way, I still killed all the dragons to maximize Faydaen's level. In the end, I finished with level 34