Sacred Tree

Well, I killed the demon, but there are still 32 vampires left. I reached Keleborn, but it seems impossible to defeat him. I read in the thread that his army doesn't grow and that he has a three-month-old army, so I was very surprised when I saw that he had 2000 Dancers, 1200 Archers, 600 Druids, 360 Unicorns, 250 Ents, and 125 Dragons.
Okay, I'll try to replay the entire map at a slower pace, taking into account all the mistakes.

Keleborn's army grows over time, unlike the demon's and the Knight's, but even the High Druid's growth can be outmatched. Overall, I can give you a couple of tips for the second and third acts:
1) You can really defeat the Knight and the demon with no losses or very minimal losses - their armies don't grow, unlike yours. This may be problematic at the beginning of the second act, so focus on other quests. The player will eventually have time to level up even more and build up an army, and spiders can perform very well in the late game if you accumulate their growth (provided that the growth is maximized).
2) Try to play the battle with Keleborn, minimizing possible losses - it is unlikely that you will be able to do it without any losses at all. You will still need the army for several post-final battles. All possible tools for this are available on the map - powerful sets of artifacts, all combat spells are available.