Various cheats

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 MissionL
     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 of the Grades, then get the Master Grade. Now the Battle
Arena will be open at Robot Beach.