Ambery
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I remember completing all the missions with a hero, but somehow I managed to complete this one on Normal difficulty. I captured the southernmost Elven town, then slightly improved it, and developed my demons. Then the enemy hero arrived, and I retreated to my own territory. The enemy rebuilt the southern town, I returned and captured it, taking advantage of the moment when the guard was weak. Then I was incredibly lucky because I encountered Gilraen with just a small group of warriors (probably during his relocation).
Then a punitive expedition arrived with an incredibly large enemy army (the Elves and Dryads alone were impressive, and there were 25 dragons, over a hundred unicorns, I don't remember exactly). I defeated them simply through incredible luck because I had the Hypnosis spell, and Gilraen's unicorns attacked his own Elves, killing more than half of them immediately. All the infantry, including the Faeries, got stuck in the town's thickets for a very long time, and their towers were destroyed. The only problem was the dragons, but I managed to defeat them by throwing all my troops at them. In general, I don't know how I won that battle; it lasted a very long time.
Then, quickly gathering my meager forces, I ran to the northwest castle and easily captured it (the map had already been cleared by the enemy).
After that, everything was easy; it was just a matter of technique