Through trial and error, testing, and making mistakes. Let's say we have a region, and when a player enters it, they will already have a certain amount of experience. Let's say the hero is level 10: we place the hero there, and depending on how they develop (warrior, mage, archer), we add a specific monster. For example, for an archer, we can add slow monsters (zombies), for a warrior, something stronger, or a few units from higher levels. Level 10 already allows them to defeat, for example, a hydra. But even that depends on whether the player has leveled up in certain buildings along the way. I, for example, completed about a third of the L-map and have already lost all the ready-made quests several times (not counting the moments with a pre-generated hero in a specific quest). If armies join the hero, it will be much easier to determine the balance.

Added 48 minutes later
Can someone tell me how to remove this [] artifact from some messages in the game?