(someone or something) to be reckoned with

A person or thing considered to be especially strong, influential, or difficult to defeat. Most commonly used in the phrase "a force to be reckoned with." Despite the higher age of its star players, they are still a team to be reckoned with. The attorney may look petite and meek, but she's a force to be reckoned with in the courtroom. Good luck getting Stan to approve these changes—that guy's a force to be reckoned with.

a — to be reckoned with (or to reckon with)

a thing or person of considerable importance or ability that is not to be ignored or underestimated.

2002 New Internationalist Now nearly 80, the ex-Harvard Pro is still full of brio and a force to be reckoned with.