For this achievement you need to freeze 12 enemies at once with the blue mage. His
skill is the way to do this. I did this at the end of the Spiral Cliff stage, as a ton of those big orcs spawn in a tight space. Note that the freeze effect seems to be quite finicky, and you may have to move around a lot to get the freeze effect on everything. Note that at max level it will come off cooldown before they unfreeze, meaning you can use it again and try to hit 12 before they unfreeze. I got it first try, so it's probably easier than it sounds.