Oxygen Destroyer

Oxygen Destroyer is the name given to the only known successful weapon that can destroy Godzilla where other man-made weapons that succeeded it failed to accomplished the same feat.

Discovery
A scientist named Daisuke Serizawa accidentally found the micro-oxygen, chemical component that would become main ingredient of Oxygen Destroyer, during experiment with oxygen element. He was horrified with the chemical reaction from micro-oxygen that can disintegrate any lifeform, either only reduce them into bones or leaving no remains. Despite this, Serizawa decided to perform some more tests, hoping that he can developed the micro oxygen for benefit to mankind instead of doomsday weapon that far worse than nuclear weaponry.

Godzilla (1954 film)
Through the experiment, Serizawa perfected the discovered element into Oxygen Destroyer, which he grimly stated that it indeed the most deadliest weapon on earth during that time (1950s). The only one whom he showed this weapon was Emiko Yamane, his girlfriend, where he implored her not to tell anyone else. Unfortunately, Godzilla's rampage to avenge the death of its family due to hydrogen bomb test that mutated him, and military forces' helplessness in stopping it forced Emiko to break the promise by revealed Oxygen Destroyer's existence to Ogata, whom hoped that they can convince Serizawa to use it.

Of course, Serizawa was furious with this and storms down to his basement to destroy the Oxygen Destroyer. Ogata and Emiko follow him down in order to prevent him from doing so. However, this only results in a short fight between Ogata and Serizawa, with Ogata receiving a minor head wound. As Emiko treats the wound, Serizawa apologizes. Ogata tries to convince Serizawa that he is the only one who can save the world.

Then, after the argument, a grim television program appears on the air, showing the devastation and deaths caused by Godzilla, along with prayers for hope and peace. Shocked by what he's witnessing, Serizawa ultimately decides to use his last Oxygen Destroyer, but only one time. Serizawa then proceeds to destroy his research, knowing that this weapon was almost as dangerous and destructive as Godzilla himself, and that destroying this weapon will be for the betterment of society.

Serizawa did used the weapon to destroyed Godzilla, but in order to ensure that he would never be coerced to revealed the weapon's components, sacrifice himself by letting himself and Godzilla destroyed by the effect of his own weapon.

Godzilla vs Destoroyah
Dr. Ijuin managed to rediscovered micro oxygen that used to be basis of Oxygen Destroyer, but instead of as doomsday weapon, he intended that the element improved for better use of mankind. When Emiko learned his invention on TV, she asked Kenichi and Yukari to stop his research out of fear that it would be as dangerous as Serizawa's Oxygen Destroyer. Though Kenichi and Yukari unexpectedly implored Dr. Ijuin to recreate Oxygen Destroyer to prevent Godzilla destroyed the whole planet due to his body now overloaded with nuclear energy, Dr. Ijuin turned out unwilling to recreate it due to the effect of the weapon can reduce Tokyo to graveyard should it used on land. To make matter worse, Dr. Ijuin discovered that Serizawa's action in destroying first Godzilla by Oxygen Destroyer turned out has created an evil, apocalyptic monster called Destoroyah.