“Flight to Salvation” – “Deeds and Paths of Darkness” [4th campaign, 1st mission]

Yes, that's right, you don't necessarily have to surrender your city....

I've heard about this mission from my brother, who played it before (on Heroes), and he said it was incredibly difficult. He told me he restarted it about 10 times, and that if it weren't for some strategic "trick" he used (which he wouldn't tell me, saying I should figure it out myself), he wouldn't have been able to complete it even now.
Yesterday, I started playing. And indeed, the mission is quite challenging. I captured the nearest city with an auto-battle against a purple faction, bought some troops... and he showed up. By cleverly destroying his catapult and ammunition cart, depleting his mana, and eliminating his air units, I forced him to retreat from the battlefield simply because he couldn't cross a wall. Meanwhile, he could have easily defeated all my units with direct attacks from his hero if he hadn't retreated; by that time, I had very few troops left...
But he retreated. I was happy, feeling like I had turned the tide of the mission (how naive I was!), saved the game, and started buying units at my leisure.
Without much thought, the second one arrived. Slightly weaker than the first, but my army wasn't exactly in great shape either... this was one of those cases where the auto-battle helps where human skill fails. I won, and I was left with 3 Gremlins.
The third time, I bought units and headed upwards.
I barely managed to capture a city.
I bought an army.
He didn't keep me waiting long... he showed up with 8 Red Dragons, 25 Riders, and so on. I destroyed his catapult, killed 8 dragons... but he had a lot of mana... a doubled reserve...
I tried to abandon the city and push further up, but there was another guy with a similar amount of mana and a Meteor Shower spell...
And they took my home city, running around like cockroaches on a table.
I hate them. Tonight, I'll try again to somehow continue... I really don't want to restart...
If you have any advice, I'd be happy to hear it...