Worshipper – “Ship” [Campaign 2, Mission 4]

Nothing works for me. I've tried 8 or 9 times, starting from the top and going down, and vice versa... Still, they "defeat" me. Please, tell me in more detail about the method you used to complete it.

I haven't encountered their heroes. I captured a demonic town, then went to gather archers for the dragons. I did this as a spy, moving from place to place. I didn't take them in castles but hired them as mercenaries on the map... When I gathered enough archers, I immediately went to the dragons... For some reason, they wanted to FIGHT! It was a waste of time, I defeated them... And then I went further and captured an empty port town. VICTORY! By the way, this was on the highest difficulty. I didn't even have a chance to fight their heroes because they simply overwhelmed me; their armies were several times larger.