Divinora
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Maybe I didn't level up the hero enough, please take a look, I've put a couple of files below the message (skills and characteristics). I couldn't find anything about leveling up the hero, maybe I didn't read carefully...
You definitely need "Chaos Magic" (the more it's leveled up, the better). The same goes for "Sorcery" (plus certain perks - I can't tell you which ones, I don't have the skill trees for the original game handy) and definitely "Magic Affinity" in "Education". These are the most important things. As for racial skills, "Arcane Wave".
Try to fight mainly with magic, and save your army. Buy leveled-up enemy heroes in the tavern. That's about it, I struggled with this mission myself, but I played without "Sorcery" (it never appeared) on "Veteran" difficulty.