Walkthroughs for custom maps.

Yes, this is the most difficult map I've ever seen. Compared to it, Хиеду is a piece of cake. The early-game timings are achievable. There's a trick involving an external dwelling with a "Explosion" spell near our Inferno Castle, guarded by 400 Hellhounds. There's a spot in the terrain where you can use three Devils, taking turns, to kill them all. The Inferno hero from the Dungeon quickly levels up magic, gets "Explosion" and "Armageddon," and casts "Forgetfulness" near the Swamp Castle. But... I can't kill Фафнер to open the tent. If you arrive early, you have few troops, and he kills you with lightning. If you arrive later, he has even more troops. The only time I succeeded was when the computer opened the green pillars near Фафнер, and I managed to grab 40 Archdevils. But... it's already too late. In short, I've never had such an interesting experience playing.

Added 15 minutes ago
I wanted to ask something. Did you use combat spells? Like distracting them to cast spells, heal themselves, or something else? What is the computer's logic? I couldn't find an answer in the physics.

On one of the Chinese maps, I fought one of the three final mega-bosses - Гирд, a hero of some incredibly high level, attacking him while he was in his castle. What's noteworthy is that Гирд didn't use almost any combat magic, but instead cast positive spells on himself and debuffs on us, even though the battle lasted for over an hour. Situations and reasons why the computer doesn't want to cast attack magic (my additional observations):
1) You attack the computer in a castle, deplete your army, summon elementals, and the computer doesn't spend its magic on them.
2) You attack the computer in a castle, and your stacks are so strong that the computer thinks its attack magic won't kill them in, say, 7-10 rounds or more, and spends magic on something else. The same happens, but not always, if you cast a protection spell against some magic, reducing the damage by 50%.
3) You attack the computer in a castle, and you only have a couple of dangerous stacks (flying units) or ranged units (but weaker than the computer), and the computer might cast only a couple of attack spells to significantly reduce the size of your stack and then start casting spells on itself.
4) The computer attacks you in a castle, without teleportation or flying/ranged units, and starts summoning more elementals, usually not spending magic on attack spells.
5) You have a lot of mana, and the computer decides it's better to defend itself.
And, as others have mentioned, there's also the case where the computer runs away.
Фафнер on this map has bracers, which you can use to your advantage. For example, you can stay in a castle and let him attack (if that's possible). In the editor, you can see that Фафнер only has four spells in his spellbook: blessing, ice bolt, chain lightning, and haste. In my opinion, you either need to let him learn something so he casts something useless (but he doesn't seem to have spells in the castle), or somehow lock him into casting these two non-attack spells: blessing and haste. Or find a way to counter chain lightning (but then he'll cast ice bolt).

Added 8 minutes ago

271200, there probably aren't any universal methods for these cases; you need to look at the situation. What was the first one?

I don't remember exactly, but I think it was Abelia by Grigori Mintchev. At the time, the map seemed very, very difficult to me. You had to go through quests, ignoring the surrounding computers, to get armies, and you had to create a quest map: what to take and where to go. I remember that there was no town portal on the map. 20% of the map was water, and you had to somehow teach the computer to walk on water, teach yourself magic by buying heroes, and there were other problems. I couldn't do it without a plan. I couldn't keep up with everything on the map. I remember that I even left my review and tips on its walkthrough in English on Meps4Heroes. Generally, the maps are passable; you usually just need to remember 3-5 directions and what to do in the coming weeks. P.S. Grigori released another map this year: Audaciousness. I started the first week, but then I found out that heroes can't get earth magic, and I didn't know how to play with that. Plus, there's a severe lack of resources and gold, a negative experience of -25,000 for the starting hero, etc. I put it aside for now. I used to email this author, and he asked for save files of a walkthrough for some map, so the author is reachable.

Added 4 minutes ago

...
the thing is, I had battles where the sequence of my actions led to the computer not using combat magic.
And I didn't pay attention to what exactly caused this.
I think that on author-made maps, there are often subtle features that are important. Like using the terrain.

A simple cheat that helps me on many maps: play the original landscape on author-made maps, and you can see what the author intended. In places where you don't see a solution or where there's an opportunity to let a hero be attacked in different locations, check all these locations. If there isn't a good one, load the HD+ mod without the SoD plugin, and voila: the landscape has changed. Repeat the check of all possible cells. This simple trick allowed me to win seemingly impossible battles by hiding a stack in the right cell and letting the computer attack you with something that can't retaliate (magical elementals, harpies, etc.). Well, everyone probably knows about the "safety" cell in Xyiedu. I found a similar cell on that map, but in a different place, and on another Chinese map (Counterattack of the Jedi), not far from the central key castle. And as for how to lock the computer into casting the desired sequence of spells, there's no universal advice, only practice. Usually, it's more or less clear when you're besieging a castle where the computer is: you destroy the ballista, lose your ranged units, kill the computer's ranged units and flying units, then cast dispel and restoration on your own (you need to know restoration :)). A battle in the field is a completely different story; here, usually, only depleting the computer's mana helps, when we hide behind a force field.