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There, one of the missions is among the easiest ones – I completed it effortlessly in about 10 minutes. I simply captured the mine and sawmill right away, visited all the upgrade locations that weren't heavily guarded – only a small part of the map was revealed. In the town, I built a Capitol, a Castle, and recruited all creatures up to the Twilight Witch. I only upgraded the Assassins; the rest remained unupgraded. On the first day of the fifth week, I bought everything I could afford – a couple of Assassins were left over. Then, I was teleported to the portal and jumped through it. The main thing is to move a few steps away from it when you're teleported to the Dungeon. The enemy heroes are scripted to a specific point near the portal, and when no one is there, they fight each other. And the remaining hero is usually defeated in an auto-battle with minimal losses.
As for secondary skills, in addition to defense (which, frankly, I don't know why he needs), I chose Chaos Magic, Summoning Magic, Education (+ Mana Attraction), and Enchantment (although I only reached the Cultist level with Enchantment). But by the end of the first mission, the foundation was laid, and by the end of the second, both magic skills were upgraded to level three. And "Summon Phoenix" saved me a couple of times in "Cultist." Although, in general, I didn't really need "Summoning Magic," and I could manage without it.