Warlock – “Cultist” [Campaign 4, Mission 3]

By restarting battles and making changes to the unit stacks, you can prevent Lethos's plague from affecting the Minotaurs, and thus reduce their losses to 15-20 in the first week. And getting the green boots in the first week makes everything much easier. Also, it's better not to fight in castles, because the battlefield is large there, and it's much harder to destroy half of the army with a meteor shower.

Of course, I restarted battles many times, and sometimes I even have to think through every move in battle. And there's also a feature in the game related to luck and morale: in one case, when a unit is in a certain position, luck/morale triggers, but in another case, if you move it slightly higher or lower, it doesn't trigger anymore. In short, it's an intelligent system, and you can't trick it :D
The green boots just wouldn't drop on my hero! I was already tired of restarting and trying to defeat the Furies and Kifru near the castle. I ran out of patience. But on the Veteran difficulty, everything went smoothly right away. And the boots appeared somewhere in the second week. Now, with Shadya and her artifacts, her spellpower is 20+, plus the boots, which equals more than 800 damage with an enhanced meteor shower. That's how I defeated Lethos. He was constantly running away from me, managing to resurrect his troops before I could cast spells on him with Shadya. She only had basic spellpower, so it was very difficult. But I finally cornered him in the castle. I had to sacrifice some Hydras, otherwise I would have destroyed all the dragons, and there were 16 of them by that point.