There are several phrases that can be used as synonyms for the phrase "person to be reckoned with." Some of these include "force to be reckoned with," "power player," "heavy hitter," and "influential figure." These phrases all suggest that the person in question possesses a certain level of authority and influence, and can be a formidable opponent or ally. Other phrases that might be used include "vital player," "key player," and "game changer," all of which emphasize the importance and impact that this person has in a particular context or situation. Ultimately, these synonyms all point to the fact that the individual in question is someone who should not be underestimated or ignored.