This achievement requires that you win 25 matches in Multiplayer. Multiplayer consists of a single game mode (Capture the Flag) and just two maps. Single-player weapons, levels, clothes, etc. carry over to multiplayer, so you may want to wait until you progress a bit into single-player before attempting this achievement. This also brings trouble as players can spend real money to get the best weapons and clothes to become invincible in multiplayer. There are only PUBLIC custom matches, no ranked or private games. Because the game is free, the online community is relatively populated at all hours of the day. The minimum number of players that can be in a match is 4 and the minimum score limit is 3. The best way to earn this achievement is to get a partner to boost with. Create a 4 player game and quickly join and start the match. The player going for the win should quickly capture a flag. As soon as it is scored, the other player should quit. The match will terminate and you will get the win. There is no counter or leaderboard that shows wins, so try to keep track along the way yourself. See the Achievement Trading Thread for other players looking to boost.
NOTE: You MUST be connected to a WiFi network in order to enter Multiplayer. A cellular connection (even LTE) will not cut it.