Can be done in either Player or Ranked matches.
You must get 25 execution kills. To do this, attack another player until they stumble backwards, LT + RT will then appear on the screen, push both triggers and you will do an execution kill.
Fastest way is to get a boosting partner, create a player match, Deathmatch mode, on the Lodge map. Set time to 5 minutes, 1 round. Make sure and kill your partner with an execution move. Your partner should respawn very close to you. Rinse and repeat 25 times