Enter the following code at the Main Title screen.
To unlock Hidden Deathmatch Mode:

Z+C

During the game hit Start, then enter the following:

Special Weapon Upgrage - Z,Z,Z,C,L,L,R,R
Max Weapons - Z,Z,Z,C,L,Z,R,C
Acquire Wingman - Z,Z,Z,C,X,X,X,B
Refill Fuel & Ammo - Z,Z,Z,C,Y,Y,Y,B
Complete Current Mission - Z,Z,Z,C,Y,Y,Y,Down, Down, Down (D Pad)
Gun Upgrade - Z,Z,Z,C,Z,Z,Z

On the 'Press Start' screen, press and hold C, press and hold B, press and hold A, press and hold Up, press and hold L.
Then release A, then C, then L, then Up.
You will hear 'Blackhole engaged' to confirm.
Now, during the game, press and hold A, C, Up, L to skip to the next level.

During play, pause the game and press:
Z, Z, Z, C, X, X, X, B