New threat.

I had 7 Phoenixes there, but they were standing in an empty spot, so I destroyed them out of boredom. The strategy seems okay – I'm not losing much.

I just checked, and the hut is still there; it hasn't disappeared. Maybe you didn't notice it because it's behind a mountain? Just in case, let me explain: the entire above-ground part of the map is divided into 4 levels. The first level is the outskirts and takes the longest to traverse. The border of each level is guarded by a large army of level 7 creatures that require resources to pass. There are only 7 ruins of a human city, and they are all (naturally) located in the Haven territory. 3 are located in the western area on the outskirts (the first level of the map) (if you follow the pointers on the map), and 1 is in the east, also on the 1st level. The remaining 3 are located in the transition zone between levels 1 and 2. 2 of them can be found before encountering the Archangels, and 1 after. You don't need to send the main hero to the first level more than once; upon returning, just complete the quest for the prophet and gather as many peasants as you need. It's better to do all the other work with secondary heroes who are sitting in prison, waiting to be freed.

Also, here's one more important thing: before entering the Dungeon, to avoid having to discard one type of unit in favor of another, the player needs to have the following types of creatures from level 1 to 7 (when upgrading creatures, they may have 2 variants, for example, Stalkers or Assassins. I don't remember all the names, so I'll denote them with numbers):
Level 1 - 3 (Stalkers)
Level 2 - 2 (Furies)
Level 3 - 3
Level 4 - 3
Level 5 - 3
Level 6 - 2 (Shadow Lords)
Level 7 - 2 (Black Dragons)
Some may disagree, but I think this is the optimal way to assemble troops for this playthrough.