Go to Xbox Live > Online Game Modes > Online Versus Play.
Select your preferred matchup (Quick Match, Custom Match, Create Session), make sure it's ranked and select your team. As soon as you get to the starting lineups, switch to the "Goalie" card, select your Goalie and press X to activate "Position Lock". Win the game to get this achievement.
NOTE: This achievement cannot be unlocked by scoring a goal and have the other player quit.
The server for this game is now closed and this achievement is no longer obtainable.