There are many different approaches one can take in sharing the Gospel. The Holy Spirit guides us in the correct approach, based upon the audience and circumstances.
The underlying motive is always love for the listener, but sometimes the message has to be blunt in order to get through. Jesus was very blunt on several occasions with the Pharisees when pointing out their hypocrisy. He obviously knew that was the correct approach to take with them.
Typically, when speaking to adherants of cults, like Mormons, an effective method is to call into question the validity of their teachers and to reveal the darker side of their beliefs.
Book of Jude, Chapter 1
17 But, dear friends, remember what the apostles of our Lord Jesus Christ foretold.18 They said to you, “In the last times there will be scoffers who will follow their own ungodly desires.”19 These are the men who divide you, who follow mere natural instincts and do not have the Spirit.20 But you, dear friends, build yourselves up in your most holy faith and pray in the Holy Spirit.21 Keep yourselves in God’s love as you wait for the mercy of our Lord Jesus Christ to bring you to eternal life.22 Be merciful to those who doubt; 23 snatch others from the fire and save them; to others show mercy, mixed with fear—hating even the clothing stained by corrupted flesh.