Worshipper – “Ship” [Campaign 2, Mission 4]

Personally, I didn't complete this mission on my first try, but only on my third. There were some difficulties. The last time, I did the following:
First, I captured the Demon City, bypassing Healing (I think that's what the city is called). After capturing the city, I gave the entire "elf" army to a secondary hero, purchased in the tavern. And I started building up my demon army. The secondary hero was responsible for mines and other things. I didn't capture the elf cities. I tried to gradually level up Agrael. I leveled him up to 25, as I didn't focus on it much and rarely ventured out. I withstood one elf siege with minimal losses, and after the siege, I bought a growth bonus and went to Healing. I should note that I had fewer losses when I immediately killed the archers and druids. I defeated Ossir and captured the city; he had a small army, and I immediately went to capture another weekly growth bonus in Erivel through the underground passage, killing the dragon, as there wasn't much time. Meanwhile, the strongest of the elves took the Erivel army and went to Healing, meaning Erivel was left with a small army, and Healing was useless to me. Moreover, she was still standing near Healing, while Agrael calmly conquered Erivel in her absence. That's all! :) Good luck with completing the mission!

Added 7 minutes later

Elf heroes keep coming to me and destroying almost my entire army. And in Erivel, there are more and more creatures, and I have to rebuild my army again and again.

Yes, I had the same problem when I played it for the first time... But the last time, I easily defended against the elves. I wrote how I completed it; maybe it will work for you. Good luck!