Access All Levels/Characters:
Goto the Selection Screen and enter the following:
Square(x4), Circle(x4), Square(x4), Select(x3)

Access Cheat Mode:
Goto the Selection Screen and enter the following:
Square, Circle(x2), Square, Circle(x2), Square, Circle(x2), Square, Circle(x2), Select(x3)
You can now enter a cheat below:

Low CPU Health:
After accessing the Cheat Mode(see above) enter the following:
Press pause and then circle+square

Instant CPU Kills:
After accessing the Cheat Mode(see above) enter the following:
Press pause and then circle+square+triangle