The phrase "in one prime" implies a state of excellence or peak condition. Antonyms for this phrase would include words that suggest the opposite, such as inferior, substandard, or deteriorating. Other antonyms might include words that suggest a lack of vitality or energy, such as lethargic, sluggish, or feeble. Additionally, words like declining, waning, or weakening could be used to describe a situation where someone or something is no longer in its prime. Overall, antonyms for "in one prime" would suggest a state of lesser quality or ability than what is implied by the original phrase.