I doubt that you will find "counter-witnessing" very satisfying. Unless they are mentally prepared to hear, it will be a waste of your time. If you insist on doing it, then you must control the agenda. Ask them to explain how the organization has falsely prophisied so many times and why they should be believed about anything. Ask them to explain the damage they have done to families by shunning and the protection of pedophiles in their midst. You could even quote them some scripture about false prophets. You might also ask them to explain their many shifts on blood matters, Rh factor, etc. and how they apologize to the families devastated by the death of a loved one.