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.
Complete Level 1 in any difficulty mode
Complete Level 2 in any difficulty mode
Complete Level 3 in any difficulty mode
Complete Level 4 in any difficulty mode
Complete all 4 Levels in 'Normal' difficulty mode
Complete all 4 Levels in 'Nightmare' difficulty mode
Complete any Level, in any difficulty, using only the Piranha
Complete any Level, in any difficulty, in a 4-player co-op game
Obtain Rank 5 in all 4 XP categories
Obtain Rank 10 in all 4 XP categories
Obtain all Awards
Within a single session, obtain at least 6 Awards in one Multiplayer round or one Campaign level.