Chat lobby matches are exactly what they sound like.
Go into Xbox Live Games, choose one of the chat rooms (usually at least one of them is "heavily" populated, with 20+ people, and just randomly challenge people through the chat room. Win or lose, after you do 100 games, the achievement will unlock.
To boost this, just make a chat room with your friend, and have one of you challenge the other. When the match is over, just have both people choose "Yes" to playing again, and you can have a re-match without backing out of the game.
This took me and MMMDI about two and a half to three hours to boost through together, but one of the good perks is, you can also boost the "Do every fatality in the game" achievement as well.
Edit : Disconnects don't count towards this achievement, so make sure you finish all 100 games. (Thanks LepraSaun)