The mission was quite easy. I immediately captured the first Elven town, bought an Elf with a special ability that adds extra damage (at this time, the main hero, with his starting army, recaptured all possible mines and took a scroll – you can guess which one). In the meantime, I upgraded the Elf's Luck and gave him a special ability that deals racial damage to selected mobs. In the end, the main hero leveled up on the large number of enemies that were scattered across the map.
P.S. After defeating the overpowered army, I sent the Elf to the upper right castle.