Hey Gamers! Welcome to NHL 19!
Video guide below if you prefer video with discussion on the mode and some tips to get this achievement.
Basically, Ones is a game mode in the World of CHEL mode of NHL 19 that takes place on the new outdoor ice rinks for some "backyard" 1v1v1 against an AI goalie. These matches take 3 minutes per match plus overtime if necessary.
You will start in Tier 4, and to get the achievement you need to play a match (not win, simply play) in Tier 1, which is played on the Diamond Rink. To advance tiers all you need to do is win 1 game, so winning one game in Tier 4 will advance you to Tier 3 and so on. You have 2 changes per Tier to advance as you will not get demoted back a Tier until you lose 2 consecutive games, so that is helpful.
Once you get to Tier 1, play the game, win or lose, and BLEEP BLOOP, we're done.