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

Gain 1000 XP - CIRCLE, CIRCLE, TRIANGLE, X
Level 2 skills - UP, CIRCLE, SQUARE, SQUARE
Level 4 skills - TRIANGLE, SQUARE, DOWN, UP
Level 6 skills - DOWN, TRIANGLE, DOWN, SQUARE
Level 8 skills - X, CIRCLE, DOWN, SQUARE
Restore missiles - CIRCLE, CIRCLE, CIRCLE, X
Three-hit combo - UP, SQUARE, CIRCLE, SQUARE
Four-hit combo - TRIANGLE, SQUARE, UP, X