There isn't much you can do because of two basic JW reactions:
1. The prepared response. The Watchtower has prepared JWs for many things and they have canned responses. It doesn't matter if they believe the canned response or not. They might not even be able to explain it in detail. They have been programmed with the response and that is all that matters.
2. The shut down. Sometimes the JW doesn't have a response. Then they literally shut down and turn themselves off. They will not respond nor will they think any further on the subject themselves.
In my experience, the one thing that shakes their faith is that they are witness to something or read something that ignites a spark. But, as many said, the spark has to come from the inside. You can't ignite the spark from the outside due to all of the defense mechanisms that are in place.
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