"Takes one on" is an idiom typically used to describe situations in which someone is facing a challenge or hardship. It can be used interchangeably with phrases such as "taking on a challenge" or "taking a hit." Antonyms for this phrase might include "avoiding," "refusing," or "ignoring." Rather than facing the challenge or hardship, an individual who employs these antonyms might attempt to shy away from the situation altogether. Other antonyms might include "supporting" or "helping," in which an individual might provide assistance to someone facing a challenge, rather than taking on the challenge themselves.