Godzilla - Kaijuu Daikessen

Play as Super Mechagodzilla and Gouten:
Hold L + R and quickly press Y, X, A, B at the "Press Start" screen. Then, hold L + R and press Up, Left, Right, Down Y, X, B, A. You can now select Super Mechagodzilla and Gouten in Versus mode.

Play as Gamera:
Hold L + R and quickly press Y, X, A, B at the "Press Start" screen. Then, hold L + R and press Y, B, X, A, Up, Left, Right. You can now select Gamera in Versus mode.

Play as Godzilla 1975:
Press Y, B, X at the "Press Start" screen.

Versus mode options:
Hold L + R + Select and press Start at the character selection screen to exit Versus mode. Hold Select and choose a character to allow the CPU to control them during the round. Press B to cancel the selection.

Hint: Spaceship:
Have Godzilla defeat Mothra with 30 seconds remaining in the round and a spaceship will fly past in the background.