The phrase "at mercy" implies vulnerability and dependence on the goodwill of others. Antonyms for this phrase might include "in control," "self-sufficient," "empowered," "autonomous," "independent," "sovereign," "dominant," "predominant," and "influential." These words all suggest a state of being in charge, able to make decisions for oneself, and free from the influence or control of others. By focusing on these antonyms, one can shift their mindset and work towards becoming more self-sufficient and independent, rather than relying on the goodwill of others to meet their needs or achieve their goals.