As a former Mormon looking back, I have found that it is difficult to persuade Mormons that another religion is true because they see things through the lens of Jesus' statement that "by their fruits ye shall know them." Thus Mormons look at their own tradition and see a religion that, depsite persecution, has experienced rapid growth and prosperity. Further, they have the strongest missionary force in the world and are one of the most charitable organizations in the world as well. Thus if you said to a Mormon "you should be Orthodox" they might respond "Why? You are isolated, don't proclaim your faith, your religion isn't growing, it isn't prospering, etc."
The best way we can evangelize Mormons is by getting our own house in order.