This achievement can be unlocked in multiplayer. Plug in a second controller. and select a character for both. Now only play with your main account. Try to make as many gems as possible. Once you make about 5-10 combinations, your character will throw a snowball.
You can also make super snowballs, these are worth double the amount of points.
To create a super snowball, focus on a character's specific gem.
For anna/young Anna this is the purple square
For Kristof/ young kristof, this is the yellow hexagon.
Make a certain amount of combinations with these gems, and you will get a super snowball. --> also an achievement for this, 3 in a round.
You will have 2 minutes to do so, my high score is 15 in only a couple of rounds played, so this is not difficult at all.