They are in a cult and have been brainwashed and manipulated. You'll have a very difficult time convicing them to see any other position but their own.
In my conversations with them in the past, I focus on tearing apart Sola Scriptura. the last convo I had with a Jehovhah witness, I told the guy to stop quoting Scriptures to me. He was shocked I told him to do that. I explained to him why. I told him, "a Southern Baptist minister would read those very same verses and tell me something the complete opposite of what you are telling me. so please tell me why you are right and he is wrong?"
He didn't know what to say to me.
I then told him, well I can tell you why I say these verses mean these things, because I belong to the Church that Christ established, of which He is the Head, and which has never left the earth or disappeared. I then told him that he can be a part of the Church as well.
That stopped him from coming back to my house again, lol