I think that you would have to prove to them from scripture that Jesus is God, nothing else is really going to work.
If you bring up the "false prophecies" I think it would be too easy to get around.
For one thing they weren't actually false prophecies, they were infact mistaken interpretations of existing true prophecies. Allow me to explain...
They used the apocalyptic books, Daniel, Isaiah, Revelations, and looked at the prophecies contained in them.
Then they tried to interpret them in light of modern times.
Then they got it wrong, and have since learnt not to try and predict the end of the world as only God knows when it will end.
If you don't like my explanation consider this...
There are a number of different views in the Christian debate forum concerning the identity of the Antichrist and the mark of the beast. They can't all be right. But we wouldn't go around accusing forum members of false prophecy for making mistakes in interpreting scripture.
So back to the Jehovahs Witnesses.
You do need to base your arguments on scripture, the more the better. And try and use scripture that isn't subject to too many different interpretations.
And most importantly, pray. Pray for God to strengthen you with Holy Spirit.