I dunno, GD, seems a waste of good study time to me. I have some knowledge of Morman belief just out of curiosity, but its very hokey and indefensible. My position is that the Word of God is the best source of reasons for the hope that lies within me. When they come to me, I let them explain their beliefs to me, and counter them from scripture. I don't know if it does any good. I always just hope that seeds are planted in the minds of those who are elect, the rest won't hear anyway.
Both schemes can yield benefit, but yes, I far, far prefer limiting discussion to what the Bible says. And let's set aside examining what the Bible says and debunk their basis for mistrusting it, first.
Unfortunately most all of them don't know the first thing about the history of the Bible. They don't know about how close existing manuscripts are to the original (fragments to within 50 years; full NT books to within a hundred years). They don't know that we have the Bible in the original language; that "translated correctly" is a complete red herring, we can read the Bible in the original language. They don't know ... basic data you know.
They see the Bible like they see the Book of Mormon -- and often like many American Christians do: a vast quantity of nice uplifting devotional stories set in a faraway land, a mythos on which to base their view of God.
Most of the time they have no real defense and shag. My take is that they're out fishing for the gullible, and they seem to quickly decide there's not gonna be any bites in this pond and move onto the next. Which is one reason among many that I find myself resisting the feeling of strong detestation for these automaton charlatans. They demean and insult Christ, and they go about dutifully netting suckers and wreaking havoc on the few elect that get temporarily caught up in it.
They are. But they are not themselves discipled to an extreme depth. At one point I engaged some of them at length over this, cranked up the volume to get their mentors involved in the Biblical discussion. And yes, they left permanently when they couldn't answer certain points. But this is the point. We have an obligation to spread this message. Their missionaries saw the basic message. They saw this was a showstopper. They knew it. I've often mentioned to God about them in later years, because they saw the glimmer of the truth. But that's the point, isn't it?
Your missionaries are doing this according to a script. Get them off the script. Connect with them like Jesus would connect with the woman in adultery, the woman at the well, or Paul would connect with people. Listen for points of contact. The Bible is a massive point of contact! And get them to investigate this "good book" for themselves and wonder whether the LDS church got it right.
I wouldn't attack the people. They have a prophet's status. Go after the statements of the church. Get their interest up in the Bible's trustworthiness. And then spring its message of the Gospel over against the Book of Mormon. Why should you listen to that source? This trustworthy source is contradicting them.
Turn the tables. Become the missionary. Win them to doubt about their falsehood. Play the harp strings on the Gospel.
I say use morman writings to wrap fish or kindle campfires, and defend the gospel from the Word.
I say work with them as springboards, especially the linchpin arguments they have to present as missionaries to every single person on the way.
"All the churches are wrong." "How about yours?"
"By faith alone is a pernicious doctrine." "Yet Paul states it flatly. How can you deny Rom 3:26-28, Rom 4:4?"
"Inasmuch as it is translated correctly." "OK, here's an interlinear so you can see what the word was originally, and what it means woodenly. How can you say it denies what it says explicitly?"
Just get one point through. Drive it like a piton into the rock face of their faith. Can you imagine how many times your points contradicting the LDS position will be repeated again and again in their minds, if you just put it there? They're missionaries! Every day they go out and have to say this stuff again & again. If by the Spirit your words cut to the heart, by the Spirit their hearts will be awake to it. And it'll chase them.