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I don't claim to be a genius player, and after 10 attempts, I still couldn't complete the 3rd mission in the Inferno campaign!!!!! It's terrible! This elf defeats me in any case, the upper town is guarded by him, the dragons defeat my troops, there's little money, and the growth is small! Oh! What should I do? Please advise! :-)
I captured the lower castle almost immediately and didn't go anywhere until the green faction cleared the way to the mines and the experience-giving tree. When the second hero arrived, I defeated him in two rounds with a "minor" hero with hellish creatures and immediately attacked with my main hero, who had an army of elves. He is highly developed in terms of movement, so I then ran from castle to castle for reinforcements, and eventually won.
The most important thing is that the green faction practically doesn't attack empty castles "without a hero" in the garrison. I periodically hired a hero who was defeated with hellish creatures, and he brought me reinforcements. Then I "sacrificed" him; he was defeated, and in a new location, I bought him again on the first day.