The most USELESS monster.

Speaking on the topic, I've never found peasants, leprechauns, and ballistas to be useful. 30 ballistas are easily taken down, but try to defeat 30 pikemen at the beginning of the game. The ballista's ranged attacker is weak. Too weak. At least the elves have 2 shots.

Peasants are good as frontline infantry – use them to overwhelm minor enemies; they accumulate quickly and have an attack of 2-3, which means 400-500 peasants is a decent force. They can even take down a 3rd-tier creature by swarming it and unloading their inventory into its back. And they provide income – that's a bonus.
Leprechauns are also weak. I used to think they were the weakest enemies – they move slowly, don't hit very hard, and their magic isn't effective – just once and that's it. But pirates are still tougher. They have a weaker attack and no magic at all.
Ballistas are needed to eliminate minor enemies. And remember – they are machines! They have no fear, no morale, no poison, no blindness, no hypnosis, and many other curses don't affect them.

Added 8 minutes later

That's what I'm saying, mobility is their best asset. Even better than stealing mana/energy!

What's the point of mobility if Holy Bolt and Plague take them down in one or two hits?