Uninvited guests.

Either I didn't understand the topic, or it's unclear how to play as a hero?
As a demon, after 3 weeks, my army consists of a maximum of dogs and a level 6-7 hero.
Then a Necropolis player with a level 17 hero and a huge army attacks.

If it's not clear how to play as a "hero," don't torture yourself. There are easier levels. For reference: the first heroes you encounter almost always have a development level close to your own main hero's level. So, if you ended up with a level 7 hero, you need to look for problems in your own strategy.