It is worth listening several times and familiarizing yourself with the arguments of these two excellent audios of Ray Comfort and Todd Friel witnessing to Jehovah's Witnesses. Both have been a great help to me and when witnessing to a JW or a Mormon, have helped to make great dents and seriously unsettle them in their faith in their false religions.
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13 min-Ray Comfort-Witnessing To A JW-"There's a Knife in my Back I've Got Three Minutes To Live".
7 min-Todd Friel-Witnessing to a JW. (From Terrified).
Other resources are this excellent audio by Charlie Campbell on The Deity of Christ. He basically takes the arguments that the JW's use against the Deity of Christ and turns them around.
It's also worth making sure you are studied up on the Doctrine of Hell (Text).
When there are two JW's on your doorstep, it is natural for us to talk mostly to the more experienced one, but instead direct your conversation and questions to the weaker apprentice.
JW: "No he didn't! Where did he say that!"
Me: "When the Rich Young Ruler went up to Him and Jesus said "Who are you calling good? God alone is good.""
JW: "How is that claiming to be God?"
Me: "Well do you agree God alone is good, it says so in the text?"
Me: "Was Jesus good?"
JW: "Well...[you can almost hear a clang in their heads] ...He was good in a different way."
Me: "...The Bible teaches He led a sinless life, perfection, how good can you get? He Himself said I AM the Good Shepherd."
JW: "The dead know nothing. So when we die we're not conscious."
JW: "It's just a parable."
Me: "What when Jesus called hell a place of everlasting torment? How can you be tormented if you're not conscience? How can you be weeping and wailing and gnashing teeth, if you're not conscious? Do you think Jesus was lying?…"
They will then nearly always try and change the subject, and quote a Scripture about something completely different, but don't let them off the hook.
Learn the how to defend the Trinity and the Scriptures that teach it. Note: They most likely won't except any argument you give so don't get bogged down on it. They may then insist the Early Church Father's did not believe in the Trinity. When you say they did they may give you a booklet to read "Should You Believe In The Trinity" (Critique Below), which is full of false quotes of the Early Church Fathers and misinformation from start to end. Do some research on this. The point I use is that the JW's say that it's misrepresenting God to use LORD instead of Jehovah for God's Name yet despite the WatchTower organisation fully knowing that it's Trinity booklet is full of blatant lies and deception it continues to use it to deceive people. I also point out "so will you continue to misrepresent God, by using it, even though I've showed you it's full of blatant lies. Because the infallible Word of God makes it clear that all liars will have their part in the lake of fire, Rev 21:8.
If you're witnessing to them on your doorstep, be prepared for them to shoot off and leave (this is normal and in their training), but if you can, make an appointment for them to come back. Although, each time try to keep them with you as long as possible, remember whilst they're on your doorstep, they're not brainwashing your neighbour.
Don't expect every witnessing encounter with a JW to be the same, as bare in mind they're all at different levels in how much they've been brainwashed. Whereas some would be fully convinced that donkey's are really from the planet Mars if the WT said so, others will be struggling with doubts. So be prepared to spend as little or as long as time with them as necessary.
Arguing about whether it's "Heaven" or the "Kingdom" is not likely to win vital ground, so like Ray Comfort in the audio above, I will use the term, "there's a knife in my back, I've got three minutes to live, how do I get into the Kingdom?"" Often they have still tried to give me a 10 minute JW theology lesson on "only the 144K" regardless, but I quickly point out I didn't say Heaven, I said Kingdom, and know what they believe regarding that.
Once to try and change the subject when she was pressed on Hell, a JW said "you've changed God's Name from Jehovah to LORD. Now, trying to win an argument on the Name of God is not going to win significant ground. So baring in mind we only have a limited amount of time I will say something like "Well the Lachish letters have proven to us God's Name is Yahweh, but that's another issue". What I am doing is unsettling them and sowing doubt and then bringing them straight back to the more important subject they were pressed on.
Everytime you get a JW into a corner, they are trained to change the subject. An example is, when hard pressed they might say something like "I'll just look for this verse... hang on, hang on... Here it is" and then they'll give you a verse absolutely nothing to do with what you have challenged them on. Don't let them off the hook.
Like Todd Friel in the audio above, tell them "1 John makes it's clear you've got to have your theology about Jesus right and He clearly claimed to be God and I'd hate you to be one of those of whom Jesus said "Many will say to Me in that Day, Lord, Lord, didn't we..." and Jesus say to you "depart from me you worker of lawlessness, I never knew you!" and send you where there will be weeping and gnashing of teeth." The objection I've encountered to this is "Well Jesus was just talking about the Pharisees" To which it is good to reply, "No, they are false converts, and they're saying "but Lord, Lord, didn't we do this work to earn salvation, didn't we do that work..." they're trusting in their own works to save them, just like you are."