Added after 16 minutes
Ha-ha! After thinking about it for a bit, a few minutes later I figured it out: the secret is in the "right" battlefield! The black dragons and hellhounds are all two-hex creatures, but we need a single position for the harpies where enemy creatures can't reach them. And the rest can even be done on auto-battle. :D To clarify: we don't need to fight this hero in this cell! We need to find (as was previously written, how and with what) a cell on the map that has a convenient position for the harpies, where they are invulnerable.
(Василь Симоненко. Люди – прекрасні)