Not enough Skeletons

I accumulated as much as I could; first of all, they didn't always give skeletons (sometimes they shoved zombies in there), and secondly, there were mechanical creatures. I took Bone Warriors.

Did you pump Fortitude?

I pumped Fortitude, but later, around level 15. Until level 7, defense didn't appear at all.

The Plague Tent is complete garbage

The choice was between the plague tent, a ballista, and sorcery

At least how he takes Dragon Utopia is a mystery to me

To me as well. Moreover, even though Crystal and Wall of Swords drop very often for Wizards, he tells me that this isn't the best way to rely on those spells. Although with Crystal you can take Dragon Utopia even from level 1 (though the turn would take about 25 minutes). He doesn't tell me or my friends anything; he just gives a link to the website http://heroescommunity.com/ and says that everything is clearly written there, basically telling us to sit and translate it if we need to.
I had Plague too, only I didn't manage to pump everything needed by level 7 + I didn't know his build and generally didn't expect to encounter him near Dragon Utopia on the 5th day. But that's how it turned out. The game mechanics and AI behavior on Hero difficulty are known perfectly by this comrade (in my opinion). He works as a programmer and plays Heroes for 10 hours a day, mostly on foreign sites. He doesn't use a scanner.

You have to kill the opponent only with a running start (left his castle in the direction of the opponent's castle and won).

That's what I tried to do, but nothing worked out for me.