Today, I spent some more time playing – there weren't any major problems; the "yellow" and "green" opponents were quickly defeated, and the blue one rarely bothers me.
I caught the "yellow" main hero with Letus and a group of seven Griffins and destroyed him through simple harassment. A few days later, Letus also captured the "yellow" Castle, because at that time, the demons released their fourth hero – I had to transfer the army to a random hero (who, conveniently, turned out to be one of the demons I had previously defeated) and quickly attack the fourth demon with seven Griffins. The battle went according to the usual scenario (the Meteor Shower spell learned in the "yellow" Castle helped a lot), and the poor demon couldn't help himself, despite his large army and level 30 – I defeated him, losing only two Griffins.
At that time, the hired demon finished off the last "yellow" hero.
As for the "greens," they disappeared quietly – I never saw their main forces because they simply weren't there – they were defeated by the apprentices from the captured heroes with small forces that happened to be in my Castles.
Now, the map is boring – there are no opponents left, only some blue demons are somewhere, but I realized that I simply can't reach them.
Before the fourth demon appeared, I was given five Phoenixes, but they are not really needed.

I am uploading three replays of the battles I enjoyed, and the battle with the fourth demon was particularly enjoyable.