The seasonal event Winter Wonderland ran until 01/02/19. It will return again in a year's time.
For this achievement you need to win a round of Mei's Snowball Offensive as the last person standing. This can be a bit tedious for an opportunity to present itself: if your team wins decidedly you won't get one and if your team gets slaughtered you may have to kill a lot of people at once. Standard Mei abilities apply but your gun only has one shot that needs to be slowly reloaded on the snow mounds on the ground using
. Enemies that have ammo in their gun will have a blue snowflake above their head so be careful, but if they don't then consider that as your opportunity to play aggressive and strike.
If you already have the 'A Couple of Flakes' achievement just hang back and see if your team is bad enough to go out and get killed. Two things that will help your chances:
-Keep your ultimate(Flurry - rapid fire infinite ammo for 6 seconds) ready in the event the opportunity presents itself.
-Drag the game out so your enemies are eventually revealed. This will ensure you don't get caught off guard and if you find yourself in a 1v1 or 1v2, Flurry will tilt the odds in your favor.
Sometimes it's just pure silliness on the other person's part.