I remember watching this show, and particularly this closing scene, on tv sometime between 1972 and 1974 (i.e. ages 8-10). It caused Giant Robot! to become permanently embedded in my vocabulary.
I also remember thinking (not precisely in these words) that if the proportion of explosion-caused-by-destroying-the-monster to explosion-caused-by destroying-the-monsters-fingernail is no greater than the proportion of the monster to its fingernail, the bigger explosion wouldnt be enough to destroy the Earth.
In other news, the following promos look generally promising: