I think this is a good explanation of what Cognitive Dissonance is, and its effect upon the mind:
"Cognitive dissonance is the term used to describe what happens to a person when he accepts multiple, conflicting information at the same time. Rather than have to deal with conflicting thoughts, a person almost "separates" himself from reality. He starts to deny anything is wrong and continues to tell people, "Everything is fine. Just wait until I get there. Then you'll see I was right." In reality, this leads to both a further detachment from reality and a further attachment to the group's values. And if a person were to question the conflicting information presented to him, he would probably be told, "Your question means you don't understand the material. Continue learning and then it will become clear." This "separation" occurs at about the same time that a person starts to lose his critical thinking..."
So, what we need to present to the JW who has lost his/her critical thinking skills is two conflicting beliefs that they hold now, and show that one cancels the other.
Pick what you like really, there are a host of them, just one, God is a god of love, yet he will destroy 7billion plus people at the Big A.