1. You need to chose the ice mage
2. You need to cast "throwing icicles" spell
You do not need to do this on hardcore (but you can if you are trying to knock that achievement out at the same time). I did this on normal. I am 99% certain you must do this on level 4, the Ice Palace. Otherwise it just makes the enemies wet. I got this the very first time I cast the spell on ice palace, after casting it 30+ time on level 1, 2, and 3. (Don't forget to save and quit after each boss, and then continue just incase your game crashes, or you lose your ice mage)