This tactic works not only with harpies, but also with any monsters with retaliatory strike and good speed against slow-moving units. (For example, fairies against zombies, etc.)
P.S. As a variation of this tactic, use walking monsters against fast monsters to "flee" and distract attention, along with a siege unit with ammunition (or a ballista).
Лидер клана "Из многих единое"
Приключения – это сплошные неприятности, только про них почему-то потом интересно вспоминать...