To unlock this achievement you have to play all of the different multiplayer game scenarios in the combat simulator. You can play either on LIVE or locally and it still counts and locally you can play against human players and/or AI (or even none at all on some match types).
By going to Combat Simulator>Game Settings>Advanced Setup>Scenario you can choose from one of:
Free For All Game Types:
Hold The Briefcase
Pop A Cap
Teamwork Game Types:
King of the Hill
Capture The Case
Complete all six and the achievement should pop.