Mei's
Ice Wall ability allows her generate a large ice wall on the battlefield, commonly used to block a pathway or as a defensive wall to shield yourself.
An ice wall is made of 5 vertical columns with 500 health each, meaning the total wall can absorb 2,500 damage. You will need to absorb about half the total health at 1,200. I recommend putting it up in front of an enemy while they are using their ultimate. Or any enemy that can generate a lot of damage quickly.
If your wall absorbs at least 1,200 damage, the achievement will unlock.
If you'd like to learn more about Mei, you can view this video. The achievement is described and shown at 10:58: