The scenario "Last Flight".

I beat it on Veteran difficulty. It will take a while to master this hero... On the first turn, I summon Imps; then, the first Imp unit joins me. If they are going to join at mines, it's not necessary. Spending 6,000 on an Imp unit at the beginning of the game is questionable. By the end of the first week, I return to the Castle, where I build Magic levels 2 and 3, and then I try to capture mines. On day 7, I build a large unit upgrade building; even so, the units are still weak, but I have the money. In the first few weeks, I don't build useless things; I focus on developing the higher-tier units and the higher-tier unit upgrade building. Quite quickly, as soon as I have some cavalry and Cave Lords in my army, I rush to the top to gain experience and collect loot on the surface, and first, I capture the sulfur mine on the right. It is guarded by Succubi; with a decent army, they JOIN. This is a key moment. I collect all the loot I can without significant costs, return for reinforcements. I completely clear my world to maximize experience gain, go to the top, explore all the areas for items and experience, and don't forget to capture enemy settlements with reinforcements. There are artifact merchants there; buy yourself a Pendant of Skill and other goodies. As for skills, I definitely take Dark Magic, Logistics, Luck, and Attack. The final battle took place on day 65. I had: 15 Devils, 30 Cave Demons, 50 cavalry, about 85 Succubi, and some other units. Miav had: 15 Archangels, 30 Paladins, etc. I drain Miav's mana with the Demons right away; she had 10 mana points (-: The hero's level was something like 18. I replayed the battle until I understood that I need to SUMMON ALL units on the first turn, not try to attack. Except for the Succubi, which I summoned on the third turn, because they are the only good offensive force, and they need to make a couple of shots right away. I cast Slow once, and then I often used the "Suggestion" spell that I got, on one of the strongest units that is closest in the queue. If you can hold out without significant losses until reinforcements are summoned, the outcome is predetermined. By the way, I placed the Cave Lords in the far corner; they stayed there until the end of the battle, casting spells, and by the end, they were the only fully preserved unit, except for the Succubi, which was very helpful.