Formal:
* Coup de génie: This is the most literal translation and is the most common and formal way to say it.
* Éclair de génie: This translates to "flash of genius" and emphasizes the suddenness and brilliance of the idea.
Informal:
* Idée de génie: This translates to "idea of genius" and is a more everyday way to express the same concept.
Other options:
* Inspiration fulgurante: This means "lightning inspiration" and emphasizes the sudden and powerful nature of the idea.
* Invention géniale: This translates to "brilliant invention" and highlights the creative aspect of the strike of genius.
Ultimately, the best translation will depend on the context and the specific meaning you want to convey.