Olympic Games: London 2012

From the main menu, go to Online Play and select either Quick Match or Custom Match (If you want to play a specific event I recommend Custom Match as it narrows down the search a little). Once you have connected to opponents & the game has loaded up, you just need to beat them in whatever event you are playing to earn yourself a gold medal. Repeat 4 more times to win 5 golds and the trophy will unlock.