Focus on John 6. This seems to do it every time--or, more properly, it seems to do something every time, and the something can be one of two things.
If you're fortunate, your discussion of that chapter--it's the one in which Jesus promises the Eucharist and states emphatically that what appears to be bread and wine really will be his body and blood--will throw the Jehovah's Witnesses for a loop. Focus on Jesus' repetition; over and over he said we're to eat his flesh and drink his blood, and over and over he failed to tell his listeners he was speaking only metaphorically--for the simple reason that he wasn't. He was speaking literally, and his listeners knew it.
First the Jews walked away, shaking their heads in disbelief. Then even some of Jesus' disciples left him, unable to accept the doctrine of the Real Presence. One particular person fell away here: Judas (see verse 64). It was here, in his disbelief in the Real Presence, that Judas first betrayed Christ. Yes, later he would be a thief and a traitor, but this is where his tragedy began.
If you go through John 6 slowly, emphasizing what's really going on, the Jehovah's Witnesses will find themselves in a pickle. You'll show them how all the people mentioned in that chapter took Jesus literally--so why shouldn't we?
If you bring the missionaries this far, end your exchange with an exhortation. Use the lingo they (and you) have heard elsewhere; they'll identify with it. Tell them they need to read the Bible. Say they should ask "Jehovah God" to give them the light to understand what John 6 means. Tell them they have to "get right with God," and let them know that means going wherever the truth leads them. Tell them they have to trust God and follow him wherever he may lead them, even if that is somewhere they think they'd rather not go.
All the above explains what happens if you're fortunate in your discussion with the Witnesses. Of course, things may go wrong--not drastically, not dangerously, but annoyingly. You may find that your consideration of John 6 produces no impression at all on the missionaries. If so, wait for their return and try again.