This is a rather simple achievement.
- Enter the second disc to access the multiplayer part of the game
- Pick 'custom match' on the main screen (the first screen you see)
- Choose the session type 'Game Lab' and leave the Matchmaking on 'public'
- Now 'search session'
You'll automatically be paired with more players. Simply complete the session to get the achievement. You don't need to win; I got second place and it still unlocked.
Game Lab is actually a pretty cool mode, by the way, so you might wanna try it out a few times. Beware: there's less XP, and no accolades and challenges to be won.