Here is the full list of all 43 Scourge: Outbreak achievements.
Complete all 4 Levels in 'Nightmare' difficulty mode
Obtain Rank 10 in all 4 XP categories
Complete Level 4 in any difficulty mode without being incapacitated
Complete Level 3 in any difficulty mode without being incapacitated
Complete Level 2 in any difficulty mode without being incapacitated
Kill 4 enemies simultaneously with the Shockwave Ability
Complete all 4 Levels in 'Normal' difficulty mode
Complete any Level, in any difficulty, using only the Piranha
Complete Level 1 in any difficulty mode without being incapacitated
Complete Level 4 in any difficulty mode
Get the killing blow on the Banshee
Get the killing blow on Lilith
Complete Level 3 in any difficulty mode
Get the killing blow on the Digger
Complete Level 2 in any difficulty mode
Kill 4 enemies simultaneously with a Grenade
Obtain Rank 5 in all 4 XP categories
Win a Capture The Flag round with 10 kills and zero deaths
Win a Team Deathmatch round with 10 kills and zero deaths
Win a Deathmatch round with at least 50 kills
Obtain all Weapons Awards
Kill 10 enemies in a row with the Manticore without dying
Kill 10 enemies in a row with the Shockwave without dying
End an enemy Player's killing spree
Kill 10 enemies in a row with the Chimera without dying
Kill 10 enemies in a row with the Melee Attack without dying
Kill 10 enemies in a row with the Guardian without dying
Kill 10 enemies in a row with the Piranha without dying
Kill an enemy Player after you die
Capture the Flag 3 times during a Capture-The-Flag round
Complete any Level, in any difficulty, in a 4-player co-op game
Revive an incapacitated Player 10 times in Campaign Mode
Kill 10 enemies in a row with Grenades without dying
Kill 10 enemies in a row with the Reaper without dying
Kill 10 enemies in a row with Adhesive Grenades without dying
Kill 4 enemies simultaneously with an Adhesive Grenade
Complete Level 1 in any difficulty mode
Kill 10 enemies in a row with the Barracuda without dying
Kill 30 enemies in a row without dying or being incapacitated
Kill 25 enemies in a row without dying or being incapacitated
Kill 20 enemies in a row without dying or being incapacitated
Kill 15 enemies in a row without dying or being incapacitated
Kill 10 enemies in a row with the Firefly without dying