if there are enough of them, then it will only take 1 turn away from them. If there are few, the infantry can. :)

Let's take nature, for example. It has many flying units (fairies, griffins, mantises, phoenixes). Even death has many (imp, vampire, dragon, devil). The monks simply won't have time to chop them all up. And what kind of infantry do you have? Please write down the creatures you recruit in the Castle of Life.

Added 3 minutes later

I think the vampire is the best, as in large numbers it can successfully defeat anyone, even Black Dragons. It's a pity that it can't handle mechanisms and elementals.

Vampires 3/week, Black Dragons 1/week. Usually, there are 3-4 Black Dragons at a normal level among the neutrals. But when you have a lot of vampires (and it's problematic to recruit a lot of them, because focusing on necromancy will lead to a hero leveling problem), then there will be a lot of Black Dragons as well. Vampires are good mainly against levels 1 and 2.