Mei's
Blizzard ultimate ability allows her to throw a small grenade type object that emits an ice storm onto the battlefield in a circular area passively damaging and freezing enemies.
We will need to freeze 4 enemies in 1 activation of the ability. As long as the enemy is completely frozen, they will count, and you are allowed to use your gun to freeze some of the enemies.
I recommend going for this while defending the payload and having multiple enemies on the objective at the same time. It can also be done in any "capture" point that has many enemies, both in attack/defense.
If you can get a total of 4 enemies frozen at once, you will unlock the achievement.
If you'd like to learn more about Mei, you can view this video. The achievement is described and shown at 11:51: