15, average level?????!!!!!!
Maybe I chose an easy difficulty, although this probably won't affect the leveling, but all my units are level 20, and some are significantly higher....
I chose the difficulty recommended in the Arandor scenario, which is the first one.

So, I also completed the scenarios for Zeta and Dark Knight (I don't think I forgot anyone).
Dark Knight has a very easy scenario. The sword and bow that give extra attack – in my opinion, it's an overkill. He only needs to level up his Warfare skill, and he can defeat everyone in one or two turns, even without an army.
Zeta's scenario is a bit more challenging, although I only had temporary difficulties with clearing out the thunderbirds (I was clearing the map very slowly, and they just kept accumulating there:D). But there are still a lot of dwellings with poisonous creatures and dark knights...:)
And the "labyrinth" on that map only left me with negative emotions,
running back and forth gets boring quickly; I spent half a month there, if not more.
P.S. The idea with hidden trees and chests (it seems to be in Zeta's scenario, and in some other scenario) is very good:)

In general, I will continue to comment as I progress.
I'm currently playing as Optimal:)