TimeShift

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.

Boosting partner needed / preferred.

Very simple. Host a player CTF match, set it to 25 captures, now simply capture your friends flag 25 times.

Find a buddy
CTF,25 captures

it is also possible to get the retrieve 50 flags achievement as well.on the last flag have your buddy steal the flag over n over while you keep refilling your energy and throwing LB+B nades at him.
keep count as well

...kill 2 birds w one stone

host a custom match on mp14 skyhook... be sure to change the settings for point limit to 26 so that the game does not end when first player gets cheevo, set the time to maximum, and most importantly, in your custom settings, set the walk speed to very fast. Take turns with your partner going back and forth between flags so that it is staggered and it will take half the time it would take if you both went separately.

although some ppl recommend doing 100 wins at the same time, there are 2 incredibly fast methods in the guides for that achievement so i would concentrate on captures.