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