I've stumped them a couple of times with a question: How can they be Christians if there offical doctrine teaches that Jesus is not their mediator? Every time their response is "Of course we teach that Jesus is our mediator," to which I say "When did that change?" They will it hasn't and I refer them to the 1979 and 1989 Watchtower articles and invite them to come back after they look that up.
None of them ever come back, I hope that I gave them something to think about.