The antonyms for the phrase "becoming soft" are "toughen up," "harden," "strengthen," "tough," "harsh," "rigorous," "challenging," and "demanding." These antonyms point towards a state of being tough and unyielding rather than the soft and vulnerable state implied by the original phrase. In some contexts, becoming soft can have negative connotations such as becoming lazy or losing focus, while the antonyms suggest a more disciplined and determined approach. Therefore, it is essential to choose the appropriate antonym based on the intended implication and the context in which it is used.