Nasos
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Each hero in the game has 255 built-in skill trees; when creating a map, one of them is selected, meaning that the appearance of a skill (including primary skills) is not random.
That's how it is; I didn't know that. I thought that only secondary skills were fixed in the trees, and primary skills were purely random.
Added 3 minutes later
The map is relatively new, from 2013, so it's possible that it was specifically designed for playing with HD+.
As an option, you can try removing some of the fairies.
I tried, but there are still 6 stacks. Although, even with 6 stacks of harpies, they can be defeated even with SP 6 (total of 3 level-ups) with a loss of approximately half of the fairies.