Godzilla Unleashed: Double Smash Codes -

Extreme Power:
Complete "Monster Island: Final Battle" in Survival mode. 

Monsters:
Complete the indicated level to unlock the corresponding monster.

Anguirus: Hedorah Terrorizes San Francisco level 
Destoroyah: Monster Island, The Final Battle level 
Fire Rodan: Biollante Attacks Paris level 
Gigan: Ebirah Swamps level. 
King Ghidorah: Mecha King Ghidorah Ravages Bangkok level 

3D models and concept art:
Complete the indicated level to unlock the corresponding bonus. Note: When the Monster Island, The Final Battle level is completed, you will unlock the 3D model and concept art for the character that you played.

Atorgon: Atlantis Rises From The Deep level 
Baragon: Hedorah Terrorizes San Francisco level 
Biollante: Biollante Attacks Paris level 
Destoroyah (NPC): Monster Island, The Final Battle level 
Ebirah: Ebirah Swamps Tokyo level 
Hedorah: Hedorah Terrorizes San Francisco level 
Jet Jaguar: Orga Thrashes Sydney level 
Manda: Atlantis Rises From The Deep level 
Mecha King Ghidorah: Mecha King Ghidorah Ravages Bangkok level 
Mechagodzilla: Cairo Is... Lost In Space level 
Megaguirus: Biollante Attacks Paris level 
Moguera: Mecha King Ghidorah Ravages Bangkok level 
Orga: Orga Thrashes Sydney level 
SpaceGodzilla: Monster Island, The Final Battle level 
The Sphinx: Cairo Is... Lost In Space level 
Titanosaurus: Ebirah Swamps Tokyo level