The phrase "secret weapon" is often used to describe something or someone that provides an advantage over others. If you are looking for synonyms, there are several terms that come to mind, such as the ace in the hole, trump card, hidden gem, game-changer, secret sauce, magic bullet, X-factor, surprise package, mystery weapon, or secret ingredient. These terms can describe a valuable resource, skill, or strategy that gives an edge in situations where one needs to gain an advantage over competitors. Using these interchangeable items in your vocabulary will allow you to express yourself more effectively and efficiently.