You have to win a match whilst not signed in to xbox live. To be disconnected from xbox live, you have several options such as:
1. Turn you wirless router off.
2. Take the ethernet cable out of the back of you xbox.
3. Take the wireless adapter out of the back of your xbox.
Then you just to need to play a match against the CPU (friendly game, 4 minutes for instance). Beat him and you'll get this achievement.