At the 'Title' screen press and hold Up and then press X to bring up the 'Password' screen. Then enter the password 031406 to gain 90,000 storepoints. This code can be entered as many times as you want.
The following monsters are unlocked when certain tasks have been completed in the game. Below is a list of the monsters and the corresponding task and how many points you need to have to be able to buy them from the shop.
Unlock Alien Mothership: As Aliens play through Single-Player mode and reach the 'Invasion' stage in San Francisco. Then destroy the alien mothership instead of defeating the enemies and the mothership level will become available for purchase at the store.
Unlock Godzilla 90's: Either buy a monster from each faction or beat the game with a monster from each faction.
Unlock Godzilla '54: Either unlock every monster for purchase or play a set number hours.
Unlock Moguera - 5,000: Unlock the Global Defense Force faction to unlock Moguera.
Unlock Mechagodzilla 2 - 15,000: Unlock the Global Defense Force faction to unlock Mechagodzilla 2.
Unlock Baragon - 15,000 Points: As the Aliens you must first free Baragon from the crystals in 'A Friend in Need' at Monster Island and then defeat him in the 'Baragon' stage.
Unlock Krystalak - 20,000 Points: As the Aliens play the 'Terran Mind-Control' stage at New York and destroy the large crystal structure with rock plates at the base which is inside the crater. Then return 9 days later and defeat Krystalak.
Unlock Megagurius - 20,000 Points: As the Earth Defenders play the 'Enter General Gyozen' stage at London and destroy 3 teal crystals. Then return near the end of the stage and defeat Megaguirus.
Mechagodzilla - 25,000 Points: Unlock the Alien faction to also unlock Mechagodzilla.
Unlock Jet Jaguar - 30,000 Points: Unlock the Global Defense Force faction to also unlock Jet Jaguar.
Unlock Obsidius - 20,000 Points: As a Mech from the Global Defense Faction play the 'Big Trouble' stage at Seattle and destroy the 3 cone crystals in the volcanic area. Then return later in the stage and defeat Obsidius.
Unlock Titanosaurus - 25,000 Points: As a Mech from the Global Defense Faction play the 'Soloist' stage at Sydney and destroy 3 circular ice structures. Then return later in the stage and defeat Titanosaurus.
Unlock Biollante - 30,000 Points: As the Earth Defenders play the 'Rumble in the Surf' stage at Tokyo and destroy the building with the glowing green base wrapped in vines. Then return later in the stage and defeat Biollante.
Unlock Varan - 30,000 Points: As the Aliens destory the crystals on the Mothership in the 'On the Mothership' stage. Then return later in the stage and defeat Varan.
Unlock Mecha-King Ghidorah - 50,000 Points: As the Earth Defenders play the 'Invasion' stage at San Francisco and defeat Ghidorah. Then play 'Mecha-King Ghidorah and defeat Mecha Ghidorah.
Unlock SpaceGodzilla - 100,000 Points: As a Mutant collect 4 Power Surges and beat the 'Tyrant' stage and SpaceGodzilla will become available to buy.