Here's why:
* "Strong" needs to be pluralized: The statement is intended to be general, referring to a group of people. Therefore, it should read "Only the strong survive".
* "Survive" is a verb: The statement is a phrase, not a complete sentence. To be grammatically proper, you could make it into a full sentence: "Only the strong survive."
More natural ways to express the same idea in proper English:
* "Survival of the fittest."
* "Only the strong make it."
* "Those who are strong will survive."
These options are more natural and idiomatic while still conveying the same meaning.