Keleborn's army grows over time, unlike the demon's and the knight's, but even the High Druid's growth can be surpassed. Overall, I can give you a couple of tips for the second and third acts:
1) The knight and the demon can really be defeated with no losses or very minimal losses - their armies don't grow, unlike yours. At the beginning of the second act, this may be problematic, so focus on other quests. The player will eventually have time to level up 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.
I replayed the map, reached the knight. Now I passed perfectly up to this point, cleared everything, collected everything on the map, didn't lose a single unit. And I came to the knight with a maximum army - 5 months, 1 week, 2 days. I have 3500 archers, 900 specters, 650 vampires, 440 archliches, 150 heralds, 140 spiders, and 130 dragons. 2 rings for initiative, 4 dragon artifacts, a staff for -20 initiative.
And the battle turns into a one-sided affair, I have a hero with 3 artifacts for initiative, level 34, and I can't even take a turn. The entire initiative bar is with the knight. In the first strike, 117 griffins kill 390 vampires! And in response, 260 vampires kill 12 griffins. Then some unreal 118 elementals with luck kill 137 heralds! I don't even understand what's happening. I have an army that is 3 times larger than his, with maximum artifacts and all the stuff in the game, and I just lose the entire battle in one go. While my army managed to cast 3 spells, 2 mass dispel and slow, and charm. None of them worked, not on a single unit.
And is it normal when a light elemental, a level 5 unit, has an attack of 75???