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in my opinion, 160-170 Gremlins would be more overpowered than the Miner. So, I think a specialist for them is unnecessary.
Regarding the monsters:
Increase the number of Gremlins to 12, the penalty can remain. Give simple Gargoyles a speed of 7, and upgraded ones a speed of 10 - this will be enough for the first turn in confrontations on grass, and 11 seems like too much.
Regarding the heroes:
The option with Halon is interesting in terms of a main hero - a mage, but I think he will still be weak. Maybe it would be better to make Ion a specialist for Gremlins.
Give Fafner armor instead of erudition, 7 Nagas with 140 hit points and armor on the second level - that would be something :D
Actually, I was thinking about who I might have forgotten... Fafner!! That's a good idea. He needs offense instead of erudition. Offense would be more relevant for Nagas.
There was a suggestion to make mages with a guild a dead-end branch, that is, after Golems, immediately allow building Giants and Nagas. Then you can put 5-6-7 in the castle on level 1.
Regarding the heroes - a significant strengthening of the Scholar was planned, so it is not a fact that replacing him with another skill will strengthen the hero.
The idea about Halon didn't occur to me. Interesting, I need to think about it. However, it might be better to take some less capable wizard, specialists in secondary spells, for example. Mysticism is still a thing.
By the way, as an option, there is the Dragon (specialist in Sorcerers), you can use him... BUT I think there might be problems with him when he is included in the game and chosen as a starting hero.
I think removing Giants from the building tree for Giants is the optimal option.