Unlock monsters
The other way to unlock monsters is to beat the game with certain characters.
Winning With - Unlocks
Godzilla '90s - Godzilla 2000
Godzilla 2000 - Destroyah
Anguirus - Gigan
Gigan - Rodan
Megalon - King Ghidorah
King Ghidorah - Mecha King Ghidorah