My plan for the next time I get JWs at my door is to ask a few questions and follow up with a scripture. Now, this is more for a short conversation and won't occupy you for hours but it's perhaps worth bringing up due to its simplicity.
My plan is to first ask what they believe regarding Jesus' return - they'll have to admit that they believe he has returned. Then I'll ask if they believe that armageddon is comming soon, they'll have to admit that they do. Then I'll ask them to read what Jesus himself said at Luke 21:8 - from the cult bible:
He said: “Look out that you are not misled, for many will come on the basis of my name, saying, ‘I am he,’ and, ‘The due time is near.’ Do not go after them.
Then I'll ask them why anyone should disobey Jesus' very specific commands not to follow after people that come in his name, claim to be him (or that he has returned and they represent him) and are saying that the end is near. I'll suggest that the combination of their doctrine and this scripture completely undermines their entire ministry and that for anyone to become a JW they first have to go directly against one of Jesus' commands.