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.
First of all, this achievement is accumulative, you don't need to go for this in a single match, it'll track your progress across your games, however, if you're boosting it doesn't take too long to get this achievement in a single game.
If you do decide to boost this and want to get it done in a single match there's a very easy method, choose to play a Deathmatch game on the "Rooftop Excess" map and set the score limit to 25, once the game has started just have the player going for the achievement stand at an agreed spot and have him hold down "B", now the second player should just keep running into him, once you've killed him 25 times the game will end and the achievement will unlock!