It's surprising that Mass Curse wasn't used in either video – it counters almost all elementals. Specifically, against Ice Elementals, the damage is 3-7, with an average of 5, but with Mass Curse, it's 2. So, reducing the damage by 2.5 times is better than an Air Shield. Or is Fire banned on the map?

Yes, as Dirty pointed out (almost a year ago, haha), Kherg was not available on the map in that region, but otherwise, it would have naturally been used.

Regarding the video link https://www.youtube.com/watch?v=7jBh4_pHJB8
I read the heated discussion about the authenticity of the playthrough.)))
Ah... those were the days!))) I reminisced.)))

The playthrough was done around 2012.
It's impossible to repeat it without special preparation and insane grinding.
Even I won't be able to repeat it, even though I have an idea of how it was done and know all the nuances. But spending a month of real time "perfecting" the playthrough – I'm not ready for such a time commitment at the moment, and since my skills are no longer what they used to be, the time commitment will increase many times over.

The difference from a normal game and offline mode is very significant and fundamental. There is a complete preliminary reconnaissance of the map/seed, skills, and hero upgrade trees. That is, we know in advance on which day our hero will be in a certain place and what skills he will need in that place. – accordingly, the right hero with the right upgrades is chosen in advance.

Add to this very lucky morale and luck rolls, as well as nothing similar from the computer. Also, lucky casts by Djinns, etc., as a result of which such improbable results are achieved... it's just save/load and knowledge of the game's possibilities.

It's a pity that detailed saves were not preserved; there are only daily logs (and that's very little).

Added 17 minutes ago

... I'm not surprised that there was even a program for morale...

There was nothing like that.
The only battle that was tricky due to random elements was on day 8. But that's the narrowest point in the playthrough, when it was necessary to get three consecutive Air Shield casts.