The most USELESS monster.

Let me get straight to the point – 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 elves have 2 shots.

I won't check the growth rates in castles and other things; I only checked the numbers that the respected Brackman posted. I set it to auto-battle:
30 ballistas against 30 elves, with the elves getting the first shot.
Result: 30 elves and 5 ballistas were killed
30 pikemen against 30 ballistas (I placed the ballistas in the far corner from the enemy, and the pikemen closer to the enemy so they could reach them faster)
Result: 30 pikemen and 10 ballistas were killed (in both cases, the pikemen were happy and let the ballista shoot first, and then they charged)
And if you put a ballista on a tower in a castle and cast some kind of magic on it :D
P.S. Don't criticize me; I'm not drawing any conclusions. I know the downsides of the ballista :D

Here. Well, in the worst case, if you're really afraid that demons might poison them or something, you can turn them into skeletons, Forest Lord, right?

Of course, you can, but there will be slightly fewer of them than the original number of demons ;)