I replayed this campaign twice and got immense pleasure. The campaign is great, except for the last battle, in which the author (as with all authors on this forum) went a bit overboard. He added so much to the final mage that the final battle turned into some kind of mess in which there was nothing left of the game mechanics or even logic. Just, let me give the mage everything... well, everything, and I'll even teach him how to use the console. I understand, of course, but the final battles are not a meeting with God, but a natural process of competition between the Player and the AI in their ability to play this game, and it is not worth going to extremes. I absolutely do not accept such mechanics that have nothing to do with the game. So I did as the campaign author did, I didn't waste my time accumulating troops, but simply gave my hero 900 Titans, which finished this final perversion. As they say, for every clever woman, there is a clever man.))
Thank you for the game.

An attempt to approach the design of the final battle from the point of view that it is a "natural process of competition between the Player and the AI in their ability to play this game" would be a failure. The player is a priori better at using the game mechanics than the current AI, and will beat it 100-0 all other things being equal. And based on the assumption that the final battle should be, at a minimum, not trivial, but, therefore, not boring for the player, the odds need to be shifted somewhat in favor of the AI (that is, in fact, remove these very "all other things being equal"). Usually, this is achieved by simply increasing the army and/or stats (and at the same time, a traditional hole in the balance is left in the form of a "strategy for accumulating troops" for the lazy). However, this is boring, unoriginal, and everyone is tired of it now, and no one will be surprised by the fact that someone "killed 69 trillion dragons with 1 skeleton." In order for the map not to be just another product from the assembly line, map makers, including me, invent new mechanics, as much as the game engine allows. And they are doing the right thing. And your post contains approximately 0 arguments in favor of changing these trends.

Plus

in which there was nothing left of the game mechanics or even logic

this is an undeniable exaggeration that has nothing to do with reality. In the battle with the mage, only 4 elemental shards are summoned each turn of the mage (if the shard from the previous turn was destroyed), and to replace the mage's troops when they die, demons are summoned once. This is generally insignificant and, in terms of the sophistication of the mechanics, is somewhere around the final mission of the original Heroes 5 campaign.