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Klivren, thank you for the detailed response, but there are too many skills to learn even half of them. For example, I just finished playing on the "Fortress of Honor" map, and my main hero was only level 9.
Now, about what I've tried:
Reinforcement. Complete garbage; it adds 4-5 units to 50 archers. I upgraded it to level 2 (it heavily depends on the strength of the army, i.e., it gives a very weak bonus to a weak army, so it won't help you break through a strong stack at the beginning).
Heroism. Yes, it's a pretty good mass ability. Why only upgrade it for the War skill? Why is Heroism for War better than Heroism for Cleric?
Endurance. In theory, it's a good skill, but the question is whether it's better than the Guardian ability.
Counterstrike. Thanks for the idea; I'll definitely try it.
Regarding magic, I tried playing by upgrading the Heal spell in the Light school. In another battle, I tried mass Stone Skin. So far, I'm not impressed. Heal restored 300 HP (to a single target), and mass Heal gave 100 HP to everyone. At the same time, the Vestals were dealing 600-700 damage. So, I'm wondering if spending 3 valuable skill points on this is a waste.
If I had to choose between Charismatic Leader and Chosen of Fate, I would definitely choose Leader; it turns out to be much more useful.
And yes, another question: what is the best skill to take for a hero at the beginning? For now, I've settled on Toughness.
P.S. Here's the build that I used to complete my last game. It may seem strange.
Hero mage with the Toughness ability.
Logistics
Tactics
Learning
Heroism II
Charismatic Leader II
Aggressor II
I took Aggressor and Heroism to increase the army's offensive potential because the defense is already quite good. And yes, it turned out to be a mage without magic. I chose this hero type only because they start with Vestals instead of archers.