Rapid fire
Select "Credits" at the options screen then press L1, Triangle, Up, Down, Circle(2), Up(3), Down.

Instant puzzle solution
Select "Credits" at the options screen then press L1, Triangle, Up, Down, Circle(2), Down(3), Up.

Infinite ammunition
Select "Credits" at the options screen then press L1, Triangle, Up, Down, Circle(2)

Infinite Health
Select "Credits" at the options screen then press L1,Triangle, Up, Down, Circle, Circle, Triangle, Square, Right, Square

One hit kill with firearms
Select "Credits" at the options screen then press L1, Triangle, Up, Down, Circle(2), Triangle, Square, Down, R1.

One hit kill with smack-jack, knife, or brass knuckle
Select "Credits" at the options screen then press L1, Triangle, Up, Down, Circle(2), Triangle, Square, Down, L1.

Expert mode
Select "Credits" at the options screen then press Down(3), Triangle, Down(3), Square, Left, Right for more challenging opponents.