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.
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Promoting also gives you 10 rating to your N7 number and 75 points to your war assets, to add to the first response.
There are pros and cons to promoting a character. However what promotion does is taking that character and converting them to War Assets.
Sure the cons are more based off if you prefer the look and weapons kit you had but is a non issue if you have it memorized. After a character gets promoted you'll need to redo your kit and armor appearance.
To clarify, it WILL NOT relock Armor Appearances. You WILL however have to change them back to the colors you once had. It WILL NOT reset weapons and mods back to level I. You WILL however have to reequip the weapon and mods of your choice.
You can promote a character an infinite number of times to gain an infinite permanent War Asset bonus to ALL Shepherds.
Once a single class reaches level 20, you can promote them. The first time you do this will earn you the Battle Scarred Achievement. If you haven't reached level 60 ether, getting them to level 20 will result in another achievement.