Your going to want to get a group of 4 or 5 gamers together and start a Ranked Match on your map of choice. I recommend Hot Coffee. If you have 5 players have the player thats going for the traitor kills stand to the side while two players each kill one of the other 2 making them both traitors. Then simply kill the traitors, toss a grenade to end the match, rinse and repeat until it pops. I dont know why the other solution says you need 15...I got it in 10 and everyone I was playing with did as well.
WARNING: If you delete your Kane and Lynch: Dead men Save game your Fragile Alliance stats will reset.