Starting in the first Godzilla film, Godzilla displayed an immunity to conventional weaponry, virtually impervious to everything the kaiju and humans threw at him. He has demonstrated the ability to survive complete submersion in magma for an extended period of time.
He has even survived being in ground zero of asteroid impacts. The only times his flesh has been visibly pierced were in battle with the Super X, Showa Gigan, Biollante, Destoroyah, and from Mechagodzilla’s weapons in the Showa, Heisei and Millennium series.
Godzilla possesses an extremely advanced and highly efficient regenerative ability.
Due to the fact Godzilla can absorb energy, particularly radiation; he can heal/regenerate much faster, more extensively, and efficiently, depending on how serious his injuries are. He was completely healed following the battle of San Francisco, while it took him 5 years to completely heal from injuries inflicted on him by MUTO Prime.
This power was a crucial plot point of Godzilla vs Biollante and Godzilla 2000: Millennium. In Godzilla 2000, it is explained that Godzilla’s regenerative abilities may have something to do with his radioactive properties, and Regenerator G-1 (“Organizer G-1” in the Japanese version) is the name given to a substance in his cells that is responsible for Godzilla’s swift healing.
In Godzilla vs Biollante, Japanese scientists use samples of the Godzilla cells (called G-cells throughout the Heisei series of Godzilla films) to help create the ANEB. Even neural tissue can be rebuilt by Godzilla’s regeneration. In Godzilla vs. Biollante Japanese scientists use samples of Godzilla cells (called G-cells throughout the Heisei series of Godzilla films) to help create the ANEB. This healing factor would be inherited by all creatures spawned from Godzilla’s DNA, those being Biollante, SpaceGodzilla and Orga. At the very end of Godzilla, Mothra and King Ghidorah: Giant Monsters All-Out Attack, after Godzilla has been completely obliterated (or so they all think), his heart is seen beating on the ocean floor, suggesting Godzilla’s Regenerator G1 would allow him to completely regenerate himself from just a heart or even a single amino acid, perhaps indicating how he survived the Oxygen Destroyer in 1954.
Godzilla’s skin is tough enough on his own to withstand most types of damage, and even when an attack breaks its hide, thanks to advanced regeneration powers his wounds heal almost instantly. These factors taken together make Godzilla practically invincible.