There was always another hero nearby, who replenished the supply of gremlins and to whom I gave the golems.
The crystal was very helpful - Zephyr places it, and the gremlin breaks it. + protection from archers (if you place the crystal near an archer, he won't be able to shoot).
Of course, after capturing the first city, such audacity was no longer tolerated, so I continued playing approximately as described above.
Added 2 minutes ago
I have a question: there was some kind of bug in this campaign, and I couldn't give the hammer to the dwarf. Can you tell me what he was supposed to give as a reward?