I find JWs have the same mentality of militant atheists. They start from a position they are right and everyone else is just stupid for believing what they believe.
When they start talking about Armageddon ask questions like
-Ask them to explain the difference between the church and Israel?
-Where does it say that everyone alive at Armageddon will be destroyed forever but everyone who has died has a new hope on Paradise earth?
-Ask about the concept of salvation though association and works and how only seven men enter into this new covenant with god. If this was god plan for mankind salvation wouldn’t be a consistent theme though the bible? (Other than twisting a parable)
Ask them if they ask answer any of those questions using nothing but a bible and get them to explain to you after why you are not worthy in gods eyes.