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1) Cheats.
2) Modifying game data (e.g., changing enemy dragons to peasants).
3) If it were easily solvable, there wouldn't be any point to the game. Nevertheless: a) gather all your forces (or the best units) with your best hero and b) attack the enemy city without a hero, and immediately retreat. The computer-controlled opponent will try to leave some of its troops in the city, and without a hero, they will fight poorly. Do this several times, so that the AI each time reclaims the city, each time leaving only a portion of its troops there. You can also leave your city unguarded or with a minimum of troops, so that the towers can fire at least once (i.e., a couple of units consisting of 1 fast monster).