Medal Completionist (20)
Collect 10,000 campaign or playlist medals.


Medals unlock if you have the settings on to see them or not to see them(To turn your medals on you must do under options while setting up your missions, then it will be called Scoring and Time HUD). For this achievement you need 10000 campaign or Matchmaking medals unlocked in total.

While you are playing you will naturally unlock medals but also a good tip is while playing career you want to kill as many enemies as you can, dying the least amount you can. Doing this you will get multi kill medals and killing spree medals.