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Oh, I really struggled with this! I replayed this mission more than 10 times. And why?
Because, following Nekrasov's tradition, I decided to play using the strategy of "doing everything at once." The result was that in the second garrison, which I, being a cool and well-leveled hero, decided to storm, I found 25 devils, 500 or 600 cerberuses, and 2,000 imps, who quickly defeated me, depriving me of mana. Only after the 20th attempt did I realize that I should go through the dungeons first, and then everything became much easier.
That's how useful the internet and forums like this can be sometimes.