While pausing, press all four shoulder buttons (L1+L2+R1+R2) and press the following buttons for the corresponding cheat:

Gain 1000 XP - X, TRIANGLE, UP, X
All special abilities - TRIANGLE, CIRCLE, X, CIRCLE
Level 2 skills - SQUARE, SQUARE, CIRCLE, SQUARE
Level 4 skills - DOWN, DOWN, X, X
Level 6 skills - DOWN, CIRCLE, UP, DOWN
Level 8 skills - SQUARE, UP, UP, DOWN
Restore missiles - TRIANGLE, TRIANGLE, TRIANGLE, DOWN
Three-hit combo - TRIANGLE TRIANGLE, TRIANGLE, DOWN (alt. UP)
Four-hit combo - X, CIRCLE, TRIANGLE, SQUARE