Trying to 'show' why someone belief is wrong is a waste of time in my opinion, and here is my reason...
You are in effect saying..hey your deluded, I am smarter than you, (and if they have been a JW or any other religion all their life) hey you have wasted your whole life on a lie.
However you sugar coat it you are questioning someone core reason for existing, I really do think that until someone start to question it themselves you are banging your head against a brick wall.
The only way i have seen is ( and only work if you are still a JW ) is to ask a question, that they themselves won't know that will lead them to hopefully to researching themselves, and kind of direct...hey look the WT is wrong here, or this fact they claim is wrong, is just gonna as already stated put the barriers up.
You are not dealing with evidence or reason when you deal with religious belief, you are dealing with emotion, and ...i think you need to wait until they question it themselves.