Godzilla was cut by Kong's axe. There was a huge gash on Godzilla's leg at the end of the movie. Godzilla is not invulnerable and his soft tissues are an entirely different matter. You telling me if Superman flies into Godzilla's brain he can't kill him from the inside? Superman is literally a bullet in this fight.
Originally posted by xPRIMEx
He still tanked the sheer force of the explosion
Originally posted by Adam Grimes
But radiation is just what powers nukes. The explosion is the main damage dealer.
They used a nuke to heal Godzilla in KOTM a matter of hours when it would have taken years for him to heal on his own, so it obviously has the opposite effect on him.