Check out the book "Captives of a Concept" by Don Cameron. Look him up in the members section on top of this site. It will help you to use the very scriptures they use and give them something to think about.
Two excellant questions posed in this book are: What is the most important scripture in watchtower theology and what is the most important date in watchtower history? Let them answer, most don't know the answer to these two very important questions.