During gameplay press START and enter any of the following codes:
Sword Upgrade:
Hold L1 + R1, Circle, Square, Circle, Triangle, Triangle, Square, Circle, Square.
Complete Current Mission:
Hold R1, Square, Square, Square, Circle, Release R1, Hold L1, Triangle, Triangle, Release L1, Hold R1, Square, Square.
Big Heads:
Hold R1, Triangle, Triangle, Triangle, Triangle, Release R1, Hold L1, Triangle, Triangle, Hold R1 + L1, Triangle, Circle, Triangle.
Unlock Battle Arena:
Earn all the Grades, then you should get the Master Grade.
The Battle Arena Will then be open at Robot Beach.
