It was very difficult. Both neighboring factions (Swamp and Elves) put up fierce resistance. For example, the Swamp, by the time I arrived on day 113, had already built up its Ducks, bought them, and was running around with them on a hero with ranged attack, who had already started to level up. And it refused to attack the Stronghold, continuing to circle the area, blocking my army for several days. I had to kill the Elves with ranged attacks using 10 Fire Elementals. And due to the buggy mountains, it was impossible to reach the third faction (and especially its treasure) at all.
But at the beginning of the second week, both Grisha and Labeta started getting Oil. I completed some random quest for 60 Lizards, cleared all the creature banks that were nearby (3 Giants and 12 Cyclops), rushed to the nearest treasure, and there I got another box with 25 Demons and three more artifacts.
As a result, a funny situation occurred. I had a lot of mixed units, mostly ranged attackers, but I couldn't even attack the Utopian (there was one, I checked after the game - it came out with 12 Cyclops). I could have broken through almost any Guardian, but I was given Black Dragons specifically so that I couldn't use ranged attackers with Oil. As a result, on day 126, I was at the peak of my power, having cleared everything. And it would have taken three days to reach the next treasure. And all this time, I would have just been standing still in terms of development.
And my opponent had 9 Angels, he could also kill almost anything, including the Utopian. But the tavern showed that my army was stronger. The opponent took a risk and lost everything on his second Guardian.