From what I know, one doesn’t have to be such a baki in Tanach, just an extra measure of common sense. And knowing Mefarshim, Rishonim etc.won’t help a bit, they’re at a different “angle”, they’re not exactly “gores” the Rambam, Rishonim, Chazal etc.”
You DO need to be very well-versed in Tanach, because THEY will quote chapter and verse by heart on Yeshayahu and Zechariah. You had better be prepared to answer correctly, or do NOT engage in conversation.
I was once approached by a J4J in college who was wearing a kippah and holding (what I thought was) a Chumash. When I opened it up, I immediately saw it was both a Chumash AND NT written in Hebrew with Sheimos in it. The person asked me if I wanted to know the “truth.” I said thank you very much I already know The Truth, and mine came before his and straight from the Source. To prevent further discussion, I turned and left.
Unless you are really knowledgeable, you cannot “win” a debate with these guys. They study evangelism yomam valayla and are fsr better prepared than most frum people. They have an answer for everything…