cofty: It's easier to challenge the Bible than to attack the organisation directly.
After many years of trial & error, I have found this is definitely the best approach and wish others would practice it, otherwise the mental defensive walls come up and there is no getting past them if your true intent is to get the other person to think reasonably.
What I also find fascinating is that if I criticize God directly and not the Organization, then the mental defensive walls do not come up and we can have good lengthy conversations. However once anything critical, no matter how small, is said of the organization, then the JW goes into mental lock down. For a long time I was in denial, but these experiments have opened my eyes that JW's are indeed brainwashed, which in turn makes me feel deep empathy for that person as I was also like them once.
I also find it funny that you can criticize God for many injustices recorded in the old testament (slavery, killing David/Bathsheeba's son, etc.), which in their mind is the Creator, Father, above anyone else, infallible, perfect in every way, and they will keep the conversation going all day long. Yet if you simply say that the JW Orgainzation has made mistakes in the past, their mind shuts off and fight or flight kicks in. Suddenly you realize they have mentally unknowingly put the Org above God.
Thank you for sharing cofty, I hope this helps others adopt a similar approach for dealing with the JW's as it has a higher batting average in my experience.
-Attewc