Pause the game, then press X, Y, X, X, Y, Y, X, Y, X, X, Y, Y. This will unlock the God Mode and Complete mission options.
Unlock the Dark Campaign:
At the episode selections screen, press X, Y, Y, X, X, Y, X, Y.
Unlock the 9320 Battle Drone:
Collect all the gold in the Light Campaign to unlock the 9320 Battle Drone.
Unlock the Earth Golem:
Beat the Light Camplaign to unlock the Earth Golem.
Collect all the gold in the Dark Campaign to unlock Fetica.
Unlock the Stone Gnome:
Beat the Dark Camplaign to unlock the Stone Gnome.