Kill an enemy with a headshot while they are being harassed by Senu (The Hidden Ones)

Finish every test chamber in Chapter 3

Reached proficiency level 10 with any weapon type

Defeated the Rakk Hive, the Vault Boss, Sledge, Krom, or Flynt in a co-op game

Sold 50 guns to a shop

Killed 25 enemies with explosive weapons

Equipped a class mod for your character

Reach level 20.

Finish every test chamber in Chapter 2

Discovered Sledge's Safe House

Earned level 10

Completed 15 missions in co-op

Earned level 5