Finding the Junkyard King:
Play mission four. Once you get out of the main building in the Junkyard and are near the ramp, do not go up the ramp. Next to the ramp is a small path that leads to the Junkyard King. He has a lot of HP and will require some time to take him out.
I have a friend - Complete a level in Co-op.
Collector - Collect all 8 hidden comic books.
Big Daddy Expert - With Big Daddy, use the flashbang to stun 6 enemies at once.
Kick-Ass Expert - With Kick-Ass, Kill 10 enemies during one rage activation.
Hit Girl Expert - With Hit Girl, Kill 2 enemies in one wall run attack.
I'm not Invisible - Set off the security camera 5 times in one level.
Completionist - Level up any one character to level 50.
Jerry Lee - Light a Piano on fire in the Penthouse.
Junkyard King - Kill the Junkyard King.
OCD - Destroy 1000 Crates.
Bazooka Expert - Complete the game in Normal Difficulty.
Slaughter Master - Kill 1000 enemies.
Too Easy - Complete the game in Extreme Mode.