For this achievement you need to capture a total of 25 flags, due to the online feature of this game being all but dead you may need to find a boosting partner, you can create a gaming session here; http://bit.ly/gTa5Tf
, you'll need a total of 2 players to boost this game.
There are 2 methods for this achievement, you can either create a game with a score limit of 1, retrieve the flag and end the game, repeating this process, the achievement is accumulative and will count across multiple games, some people prefer to use this method due to this achievement; TimeShift Champion in TimeShift
which requires you to win a total of 100 multiplayer games.
Alternatively you can choose to set the score limit to 25 points and get this achievement in a single game, if you decide to do this it's also a good idea to set the time limit to the maximum, it won't take that long but you also don't want it cutting out while you go for this.