Yes, I assumed that I needed to go through the portal and capture some cities there. It's unclear how Haghesh will help me defeat a hero with units that have 50% resistance to arrows and chaos magic. But that's not the point, I'm too lazy, maybe I'll try exploring the portals later.

You buy a second Orc hero, give all the troops to Haghesh, wait a day for reinforcements from the three centaur dwellings to arrive, plus build a centaur dwelling in the castle, buy them there, and in general, Haghesh will have so many archers, including those from the tavern, that the AI will be overwhelmed. Of course, before the battle, divide the stacks so that the AI's attacks are diverted by a ring of ice, not on the archers, but on the goblins or orcs gathered together. With this setup, victory will be achieved with minimal loss of valuable archers.