I recently tested a battle - https://www.youtube.com/watch?v=Lq9nI-nTEQU&feature=youtu.be
It's possible to win even with 2 harpies, but it depends on the damage roll, so I decided to leave only one, in which case we don't take any damage at all.
This only works if the harpy occupies the central cell; the problem is that you need to manage to make it do so. It also worked this way, but it's more complicated: https://imgur.com/a/muiuf
In principle, this can also be considered an exploit, but if you fix everything, the game might not be as interesting =)
Added 42 minutes later
Ah, I understand. Fear of exposing two stacks to the meteor shower.