For this achievement, you need an enemy to actively be using an ultimate ability and do enough damage to freeze them. Someone getting their ultimate does not occur all that often so with that being said we are looking for the enemy to have a D.Va as our best chance. De-mech the enemy D.Va (do enough damage to her that she gets out of her mech suit) but don't freeze her once she's out. Keep an eye on her and hopefully she will recharge her mech. When she calls it in to reenter it, she is low HP and standing in place. Damage her enough to freeze her and this achievement is yours.
Another option that occurs are an enemy Roadhog ultimate. With his heal he will charge quickly but due to a high HP pool he isn't the best option. However, keep an eye out if he goes whole hog do your best to kill him. Keep in mind crowd controlling him (think McCree flashbang or Anas sleep Dart cancel the ult) so avoid using those on him if you are looking for this achievement.
Most of the other characters don't charge their ultimate's fast enough before the round is over to give you a shot at this.
*As an edit, as of 12/21/20, its has been found that killing yourself while ulting as bastion or Pharah face the wall and shoot to kill yourself will net you this achievement. This is assuming you can charge the ult with one of the two. This is another option if the above is not working. (Noted in the other guide here) (play tested to ensure working conditions)