One of the ways to acquire the “Battle Scarred” achievement is to “Promote a multiplayer character to the Galaxy at War”. What does this mean?
Once your multiplayer character reaches level 20, you can choose to “Promote” them. It’s a bit comparable to the Call of Duty series’ “Prestige” system, in that your multiplayer level for that class is then reset to 1, and you level up again to repeat the process.
Promoting imports them into your single-player game as a “War Asset” worth 75 points, which goes towards increasing your Total Military Strength. In addition, playing the Multiplayer maps will add to your Galactic Readiness multiplier. This functionality is what BioWare designed in order to have multiplayer affect your single-player game.
Promoting also adds 10 to your N7 multiplayer rating.