Here is the full list of all 20 CO-OP : Decrypted achievements.
Unlocked a costume!
Completed a level with a box!
Collected ten orbs!
Completed a level without dying!
Used BLU3's power!
Used R3D's power!
Collected five orbs!
Got caught on camera!
Collected your first orb!
Noticed your friend's disappearance!
Collected all the orbs!
Completed the game!
AFK'd for 5 minutes! Back to work slacker!
Reached chapter two!
Reached chapter three!
Beat the best time in one level!
Completed the game without dying!
Completed the game in less than an hour and a half!
Completed the game on Purist!
Blew up your first turret!