Roller Champions

To unlock this achievement you need to navigate to Skater Tab and select rare and above items (Sky Blue, Purple, Gold) for the four Outfit slots. Once all equipped you just need to win a game ! Description is a bit misleading and makes you thing that you need signature and / or emotes as well.

Here is a visual representation as well:

You only need rare gear in the costume slots. So outfit, gloves, skates and helmet. No emotes required.

Lootboxes can be gained weekly from some sponsor contracts or through the Battle Pass (some even on free tier as well) so getting the gear shouldn't be too much of a problem. Of course you can drop some real coin and buy sets from the store if you choose.

Also complete Ubisoft Connect quests to earn points for a rare pair of skates through their in game app.

Should all come naturally when working towards 200 wins etc.