Mr. Freeze's Ice Gun is activated by holding down the B button. Mr. Freeze is unlocked by playing the second level of the first Villain Episode, which you can access after completing the first Hero Episode.
You must be careful when trying to freeze enemies, as if you are too close Mr. Freeze will pick them up by accident. You can choose to put them down with A and try again, or smash them with B, which will put credit towards the Achievement for killing X enemies with Superstrength.
If you play through that level as Mr. Freeze, you will most likely get this Achievement; however, if you are playing with a friend, you won't get it unless your friend doesn't kill any enemies until you get this Achievement.
If you don't manage to get this within the level, you can either replay the level, or any other level in Free Play. The other option is to go anywhere in Arham Asylum and freeze the goons, henchmen, and other characters that are walking around, as they respawn continuously and you won't have to worry about losing studs.