Full list of all 12 Scourge: Outbreak achievements worth 400 gamerscore. It takes around 20-25 hours to unlock all of the achievements on Xbox 360.
Obtained all Awards
Completed all 4 Levels in 'Normal' difficulty mode
Completed all 4 Levels in 'Nightmare' difficulty mode
Obtained Rank 10 in all 4 XP categories
Obtained Rank 5 in all 4 XP categories
Completed Level 3 in any difficulty mode
Completed Level 4 in any difficulty mode
Completed Level 2 in any difficulty mode
Completed any Level, in any difficulty, using only the Piranha
Within a single session, obtained at least 6 Awards in one Multiplayer round or one Campaign level.
Completed any Level, in any difficulty, in a 4-player co-op game
Completed Level 1 in any difficulty mode