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Hmm... as far as I know, on Hero difficulty, the AI prefers to attack archers and flying units first, rather than infantry, even units like harpies. And they die quickly because there aren't many of them and they have low defense.
But for some reason, on Hero difficulty, the AI still attacks harpies... Most likely because there aren't really any archers (I take hunters at level 1, and then Night Stalkers, and if not Night Stalkers, the AI still doesn't seem interested. Probably because archers are weak). And it rarely attacks dragons...
As a result, harpies always die:(, even though they are excellent units.
Especially when you run out of the castle gates, attack, and run back in, hehe;)