Fairly straightforward, just win 25 CTF games with 2 people on blue team and 2 people on red team (make sure they've all joined their team when you capture, or it won't count). Use the Little Trouble map and set the score to win at 1 capture. This can be done even faster if you use the console command "samRestartMap ()" without quotes to instantly restart the map and skip the post-game countdown.