No problem, the hero should have the creature summoning skills upgraded. I chose Air and Dark magic. You take a main hero with a minimum number of creatures and keep pushing towards the Castle located in the upper part of the map. Don't forget to attack bots along the way to collect resources. Two heroes with upgraded movement and resource gathering skills will collect them. Don't forget to replenish mana along the way. In the very end, or a little later, you approach the enemy Castle. The battle scheme is simple: send a Dark Elemental to the ranged units, choose whichever one you prefer, and there it is, the red flag – he's tough, and everyone attacks him. On the second turn, summon two Air Elementals under the gates. Then, I use Subdue, Chain Lightning, etc., and the Fortress falls easily.
And then the most interesting part begins. Leave the hero in the Fortress; let him sit there and regenerate mana. There's also a flag there, and then dogs start coming from the Underworld. The scheme is the same: Elementals and finishing them off. But here's an unpleasant moment: two enemy heroes attack your Fortress, and you meet them with a purchased hero, upgraded for Elementals and magic. Don't forget to give him the Staff of Dynasty; he simply tears apart the newly arrived hero in the Fortress.
In the first captured Fortress, 4-5 enemy heroes will come to kill you. Then, you go through the portal yourself; there's immediate mana regeneration. And that's it, I think the first Fortress in the mission is defeated. Then you are thrown onto the surface and continue to defeat enemies. After that, it's all boring because there are no more opponents.