Witnesses are told not to waste time on those who are not as they say 'Meek and Teachable',or in other words,potential recruits.
When I was a witness someone who was never a witness possed as a potential recruit but said to me ,'I would be happy to become a Jehovahs Witness if I was absolutely sure it is the truth,but I have these serious issues that I need some help with.'
This approach was not threatening to me as a JW and this guy proceede to tactfully help me on the way to recognising I was in a cult,he had done some good research and used the Watchtowers own material to show the double talk and let me draw my own conclusions.
Try not to get bogged down on discussions on the trinity etc,the main point in any discussion with a JW should be to get them thinking as an individual and not parroting answers as part of a collective,and to dismantle the illusion they have of the Organization equating to God,and to see it as having no Divine standing,but as just a load of men who have taken themselves to seriously,but you need to be quite tactfull how you go about it.
One common tactic I used as a JW was that when a subject got on sticky ground I would try to change the subject,For example on a subject of say,has the Society proved itself a false prophet,a witness may suddenly say ,'but we dont fight in wars',but you will have to get back to the subject in hand and not let them do this by saying something like ,neutrality is a good subject,but we are talking about prophets and I think you would agree that it mkes no difference whether a false prophet is pacafist or not,he is still a false prophet so we must clear this subject first.