The following suggestions, written after years of experience by John L. Smith, are adapted from his book, "Witnessing Effectively to Mormons".
"The biggest problem we face in dealing with a Mormon is in 'getting him lost,'" writes John L. Smith. "He does not feel that he is a sinner, rather, he is a god in embryo. He believes that 'As man is, God once was, as God is, man may become.'"
"That God has parents, maybe a thousand wives, that every man can become a god and might attain the position God now has, that God married Mary to produce Jesus, then gave her to Joseph to be his wife during the "mortal state" and may, "after the resurrection, again take her as one of his own wives to raise up immortal spirits in eternity." (Orson Pratt, The Seer, page 158.)
"He doesn't believe the Bible without question. He reserves the right to consider any passage 'mistranslated' that he cannot make teach what Mormonism professes to believe."
"He believes that Christ and the Devil are brothers. Early Mormon apostles taught that Christ was a polygamist."
To start with the Bible, using traditional witnessing methods, will get you nowhere. You must first turn his own books against him, and make him question the teachings of his church.
Smith asks the Mormon, "What is your concept of God? Do you believe He has a body? Do you believe He had a father, mother, brothers, sisters, wives, arms, legs and a body?"
"Do you believe 'As man is, God once was, as God is, man may become?'" Smith says, "most all Mormons know this couplet though they do not usually know its source."
Smith then asks, "Does your book teach that God and Christ are two Gods? Is he progressing?" (They would answer yes.)
"Surely your book, The Book of Mormon, teaches what you believe about God...Show me; show me one verse in your most distinctive book that teaches what you profess to believe about God."
Mormons believe The Book of Mormon teaches with "much greater clarity and perfection that those same doctrines revealed in the Bible. Anyone who will place in parallel columns the teachings of these two great books on such subjects...will find conclusive proof of the superiority of The Book of Mormon teachings..."
"Take away The Book of Mormon and the revelations and where is our religion? We have none." (Mormon Doctrines, 1966, page 99).
But instead of supporting these Mormon doctrines, The Book of Mormon will refute them. Smith offers the following list of The Book of Mormon references which will declare that God is spirit, eternal, unchanging, and that the Father, the Son and the Holy Ghost are one (contrary to Mormon teaching):
1) The testimony of the Three Witnesses concludes with "And honor be to the Father, and to the Son, and to the Holy Ghost, which is one God."
2) "And I spake concerning the convincing of the Jews. that Jesus is the very Christ,...that Jesus is The Christ, the Eternal God." (2 Nephi 26:12)
3) "For do we not read that God is the same yesterday, today, and forever and in Him there is no variableness neither shadow of changing?" (Mormon 9:9)
4) "For I know that God is not a partial God, neither a changeable being; but he is unchangeable from all eternity to all eternity." (Moroni 8:18)
Once you have shown him that his own Book of Mormon does not teach what he believers, you can point out that you would have to reject his doctrine if you believed his book, or reject his book if you believed his doctrine.
The purpose of all this is to show that Mormonism cannot do what it claims, that those who trust it are left as sinners, lost, and in need of the only One who can provide the "resurrection" that all Mormons wish to have.
Jesus said He is the only way (John 14:6) and no one can come to God but through Him.
Remember -- before you can show a Mormon he is a sinner, you must show him that Mormonism is not what it claims to be! Only then can he believe that he is lost. You can then present the Gospel of the Bible as you would to any other lost person.
Space here does not permit the wealth of examples and wisdom of Smith's book on witnessing to Mormons. We highly recommend you read it through before you start.