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Okay, if it's happening with single units, then it's probably not a bug. It was designed this way, so you can't just use single units... Also, water elementals won't freeze a much stronger unit with their attacks...
Not long ago (but before version 1.2), 100 large demons (I think they were called Destroyers) were killed by 3 water elementals and 30 archers.

Барон "Vampires"
Отдай свои прошлые сны,
В сомненьях себя обрети,
В бешеном танце душа кружится!
Не думай, что будет потом,
Здесь и сейчас мы живем,
Стань хоть на миг сам собой, слышишь!