Finding the Junkyard King
Play mission four. After getting out of the main building in the Junkyard and you are nearing to go up the ramp, do not go up the ramp. Next to the ramp is a small path that leads to the Junkyard King. He has quite a bit of HP. It will require some time to take him out.
Complete the following tasks to unlock PlayStation3 trophy rewards.
I have a friend (Bronze): Complete a level in Co-op.
Collector (Bronze): Collect all 8 hidden comic books.
Big Daddy Expert (Bronze): With Big Daddy, use the flashbang to stun 6 enemies at once.
Kick-Ass Expert (Bronze): With Kick-Ass, Kill 10 enemies during one rage activation.
Hit Girl Expert (Bronze): With Hit Girl, Kill 2 enemies in one wall run attack.
I'm not Invisible (Bronze): Set off the security camera 5 times in one level.
Completionist (Bronze): Level up any one character to level 50.
OCD (Silver): Destroy 1000 Crates.
Bazooka Expert (Silver): Complete the game in Normal Difficulty.
Slaughter Master (Silver): Kill 1000 enemies.
Too Easy (Gold): Complete the game in Extreme Mode.
Additionally there are two secret trophies.
Kyle Foddrill and Draigh Vultinide
Jerry Lee (Bronze): Light a Piano on fire in the Penthouse.
Junkyard King (Bronze): Find and kill the Junkyard King.