I’ve worked with Jews for Judaism, and others, against missionaries in person and in online environments. It takes a thorough knowledge of nach, and, in my case, a fairly detailed knowledge of Judaean history circa 150 bce to 150 ce, as well as church history and the history of antisemitism. As well, a working knowledge of the most troublesome texts (for them! all troublesome for us) in their “new” testament. This is sufficient for a trained and knowledgeable counter missionary to occupy the time and efforts of a missionary (the point, so they don’t get their teeth into someone who is unprepared), and sometimes, even to succeed in bringing a lost “messianized” Jew back into the fold. (I haven’t had this zechus, but one of Jews for Judaism’s founders, Julius Ciss, is a friend and you can find out about his journey on the Jews for Judaism website.)
But what anyone at Jews for Judaism will tell you is that the best defense against missionary activity is Jewish learning, Jewish knowledge, and Jewish identity. THat is why it is so important to be marbitz Torah l’rabim, especially in communities where there is little connection to the larger kehila and its institutions.