The fourth stage of the "Van Kanighem" tournament.

The details of who cast what and when are not essential. If I understand correctly, the idea is that by repeatedly reloading a save, you can make the Djinns and Mages waste mana on various things.

Not exactly. By repeatedly reloading a save, I made it so that the Djinns tried to cast various spells on the hero, but due to the initial Magic Resistance skill, he resisted them, and they wasted their mana. To do this, I attacked with a single thief in front of the hero, so the hero was not in direct line of sight for the mages and Djinns' offensive spells. On the first turn, I summoned 10 sprites, and they also stood in front of the thief. At this point, the mages and Djinns start exchanging "pleasantries." The mages cast Haste on themselves and the Djinns, and the Djinns cast Bless on themselves and the mages. And they don't use offensive spells on the weaker units. They spend two turns exchanging these buffs; on the first turn, I summoned the sprites, and on the second, I attacked the mages with the Fireball. The mages, in response, cast Poison on the hero. In this battle, I got tired of repeatedly reloading saves – the mages cast Poison, and that's it.
And for this, the hero doesn't need to be in direct line of sight. Well, okay, I agreed to fight while poisoned. Besides, my hero was already quite strong in terms of health, and he had already drunk a potion of immortality. After that, the Djinns started trying to cast various spells on me; I attacked with the Fireball. If any of them tried to attack the sprites, I summoned new ones. In the end, when there were only a few mages left, they poisoned them for some reason. In the end, at the end of the battle, I had 10 newly summoned sprites and the thief remaining. I sent in the hero at the end when the Djinns were out of mana; I managed to slow them down and weaken them with the Fireball, and I had already taken care of the mages earlier.

And for this, it is necessary that the hero is not in their line of sight...

Absolutely correct.

...or how (and how to provide cover, as there are no units in the army, otherwise they will target them)?

I wrote about the cover – it consisted of the thief and the summoned sprites. Apparently, they don't want to waste offensive spells on weak units. Maybe this isn't always the case, but I think you can achieve this by repeatedly reloading a save.

Otherwise, I got the impression that the Djinns attack with Ice Bolt at the first opportunity, and that's it. And here, they need to cast it 8 times.

Absolutely. As soon as the hero shows himself, he gets hit with offensive spells.

Here is a general question: under what circumstances do they not use damage-dealing spells?

I can't answer this question with absolute certainty. But I think you need to protect the hero with weak units. In this battle, this trick worked great. But sometimes they might attack the cover. In short, if the mages and Djinns started attacking the cover continuously, it wouldn't be good. I was hoping that the unit was well-leveled in terms of health, had +20 armor, a +50% health ring, for a total of 300 health. Well, I thought that if necessary, I would buy more potions of immortality. Option number two is to not defeat them all at once – a repeated attack tactic. But in my Walkthrough, this was no longer suitable – the hero had little time left. In short, I got lucky and managed to win with minimal losses. :)
I can upload a save before the attack later for testing if needed. I just have half of my saves at home and half not at home. I will probably have this batch of saves at home only tomorrow.
If you need saves before any battles, write.