I tried different approaches, used various heroes, and confused the AI. I led the main hero into a portal and gathered reinforcements, sold artifacts. My advice: create a random map, open it in the map editor, and look for the "Cartographer" object for both the Dungeon and the main Castle. Place them near your castle, save the map, and from the start, reveal both maps. When creating the map, specify the castles that the enemies will play as and exclude Necromancers (so they don't use "Veil of Darkness"). This way, you'll see how the AI plays, what it does, and what it doesn't. This will help you gain experience. In the map editor, place 7 valuable resources of each of the 4 types, 15 wood and ore, and 30,000 gold on the ground, and set the most difficult mode when starting the map. You'll have an equal amount of resources and money at the start compared to the AI, but it will play at full strength, and you'll learn.
There's also a specific algorithm for building structures in each city. When creating the map, always choose: Very large map, with a Dungeon, no water, strong monsters, 8 players, no teams. If you like the "Dungeon" castle, choose Gunnar as your starting hero. Here's the building algorithm: Day 1 - Town Hall, Day 2 - Harpy Nest (upgrade the building immediately - this is your most powerful unit for the first 2 months), then combine armies and give Gunnar only the upgraded harpies, and give the other heroes troglodytes and cyclops. Day 3 - Level 1 Mage Guild, Day 4 - Forge, Day 5 - Market, Day 6 - Stronghold, Day 7 - Castle. Week 2, Day 1 - Mage Tower, Day 2 - Capitol - all economic issues are resolved. Then, hire harpies and a new hero with harpies, add him to Gunnar. Day 3 - Resource Stockpile (we're gradually moving towards dragons, accumulating sulfur). Day 4 - Cyclops Lair, Day 5 - Medusa Lair, Day 6 - Level 2 Mage Guild, Day 7 - Manticore Lair. Week 3 begins, and this week we build dragons! Don't rush to upgrade, save up money so you have enough to hire cyclops, manticores, dragons, and harpies. Medusas are usually useless in the army; their damage is almost the same as cyclops, and sometimes even less, and they are 1 square slower. If you're starting to break through the "Snake Nest" and "Griffin Conservatory," you need 2 slots for new units. Remove troglodytes and medusas.
So, your army of cyclops, medusas, and troglodytes is transferred to the second hero. Troglodytes are divided into groups of 1 (double-click on them in the hero's window). Use troglodytes to protect cyclops and medusas. Gunnar's army consists of upgraded harpies and manticores. Then, you break through everything you can. If it's difficult with harpies and manticores, add cyclops, medusas, and troglodytes. Don't forget to upgrade the troglodytes, but don't hire them; just transfer and upgrade them with other heroes. It's more profitable to buy a hero with an army than to hire them. Sometimes, warriors and ballistas will be cheaper with a hero than buying them separately. Also, having many heroes means scouting and a chain for transferring the army from the castle to the main 2-3 heroes, or exchanging artifacts and monsters between them.
Another trick: when attacking the "Snake Nest," divide the troglodytes into groups of 1, creating 4 squads, and leave 3 squads of manticores, medusas, cyclops, or minotaurs. This will reduce the loss of important harpies or other units, as the AI usually attacks the squads in which it can kill at least someone. Another trick: you can wait for the enemy to approach and send one troglodyte against him. He will die in response, but the enemy will no longer respond, and then you can attack him with the entire army without a response. This is how I defeat huge enemy armies. I often place a "looter" hero near the main hero, who carries 20-30 level 1 units.

Good luck! Write if you have any questions!
I look forward to your comments and suggestions!