You need to develop the hero to the maximum.

10 is the maximum level; after that, the story and scripts don't progress.

Judging by the logs, the archers killed only 64 ghouls, but you still have 800 of them.

This is the beginning of the battle.

Defend, wait for them to attack you, eliminate the archers, heal, and resurrect your troops.

He doesn't always leave the gates as long as he has archers alive. And if he does leave, it's one stack at a time.

That's what I was doing. I was shooting at the archers with my own archers + agony on them + withering on my own + necromancy. But after a few turns, the catapult destroys the gates, and they ALL attack me. 2 elite + 3 main stacks against my 2 main stacks, even though they are numerically superior, win (I'm not counting the archers, they are fighting each other).

If you lose, you can buy back your hero and troops at the altar and attack again.

That's an idea, to not let him buy more troops and just overwhelm him with numbers. Are there any other options?