This is a tricky post to reply to because there is no "right way" to approach this. I have spoken to Mormons on 3 different occasions all 3 of which were about 3 hour meetings. It all started with me going on mormon.org and speaking to a missionary via chat based on a question i had. They asked to come by and talk and I said sure. Soon they sent missionaries to my door and we had long discussions. I would say a few words to you if you do get into conversations with Mormons or any cult. First, it is never wrong to speak to anyone regarding the truth so your zeal is excellent. On the flip side it is important to be wise and mature about it knowing how to handle them and when to call it and leave it in God's hands. Mormons are human beings like you and I. The reality of it is that you need to focus on prayer and make sure your soul is well aware that this isn't a fun theological exercise. What I mean by this is focus on them and love them, spend time realizing both the wrath that they kindle in Jesus for mutilating His truth and the grace God has upon them in spite of doing so. I say this because Mormons have an agenda and that is to come to your house and walk you through their beliefs. It is really easy to get into multiple topical debates and miss the fact that being lost is a reality and that it requires the Holy Spirit to perform a miracle in their lives.
A conversation with a Morman can be a complete misunderstanding if you are not aware of their beliefs and their redefining of our common terms. I think it is important to focus on passages of Scripture they bring up and give them a lesson on exegesis in every place they quote. Now again, the more you talk the more you will realize they are on a different planet in what they believe, but again remember it is the Holy Spirit who does the real work.
The way that my conversations have ended were me basically being up front with them and telling them that they were wrong and warning them of what they were doing. I don't know for sure, but I'm convinced that Mormonism was born out of a lost Joseph Smith's attempt to save his brother after he had died. If I remember correctly Protestantism's view of eternal hell had left Joseph smiths brother condemned (in Josephs's understanding of the Bible and his brother's past) and I think he somehow created this cult as a response to a merciless mean view of God and unfair view of eternal torment. For example: Historical Protestantism had a big question: What happens to lost people in other countries in the first century after the New Covenant began after Jesus work was complete? Answer: God is just in sending the lost to hell even if they don't hear the gospel. Mormanism's response: Jesus went to the Americas and met the Indians.... Another Protestant hard truth: Do people get a second chance after they die to say yes to Christ? Answer: NO. Mormanisms response: yes people do get a second chance. I could go on and on, but again the focus is the Scripture and prayer if you encounter a conversation with Mormons.
If you are already well aware forgive me for being lengthy
