Battle Scarred Achievement in Mass Effect 3
Promote a multiplayer character to the Galaxy at War or import an ME3 character.
How to unlock Battle Scarred
SP: Importing an ME3 character can obviously not be done if you have yet to finish the game with your ME3 character. This means, you must beat the game and with that save file, you can then import. This is what people will get when starting their 2nd run through of the game if they never imported. Other players must just start a new game and import.
MP: You must play enough to get one of your characters to the max rank (level 20). After doing this, you will be able to promote them.