All weapons
Pause the game, and then press: Up, Right, Left, Down, B, L, R, A, Right, X, B, Left, R, Y, Up.

Invincibility
Pause the game, and then press: Down, Left, Up, Right, L, A, X, Left, R, B, Y, Up, L, A, Down.

Full pause screen
As with most other Dreamcast games, hit X+Y at the pause screen to remove the menus from view.

Gameshark Codes

God Mode	1944170E00000001
No Clipping Mode	6DAB6ADE00000001
Homing Attack	91A80B6800000001
Debug Display	E9071AB600000001
Super Jump	88B38C9E0000001E
Super-Duper Jump	88B38C9E0000003C
High Jump	88B38C9E0000000F
Tiny Character	F1A1971800003F80
Giant Character	F1A19718000041A8
Narrow Character	57BF76C900003F80
Wide Character	57BF76C9000041A8