This one is a bastard, specifically:
Played over 1000 Ranked Multiplayer Matches (Secret)
You'll get this one before the infamous "5000 games" achievement, but to get it fastest, play tourney quickies. Each game only requires 2 frags to win.
Alternatly, party up with someone, (using the ingame party system) and they can start a game and frag you while you go eat, take a shower, get a slurpee, etc...and you'll get credit for each game played, win or lose.