Being a former Protestant myself, the biggest thing for me was finally addressing history. Because once you see what the earliest Christians believed you can't remain a Protestant for much longer.
The Protestant viewpoint is inverted, meaning they start from today and work backwards rather than starting from "ground zero" (e.g. Pentecost) and moving from there to today. From their view, it's only natural to be a Sola Scripturist, because they don't have an historically rooted understanding of their faith. They approach Christianity as a belief system, a set of doctrines and theoretical positions, rather than a mystical communion with Christ in His Church. This affects everything. And thus, this false foundation is what has to be addressed first.
Ask questions like, "Have you ever read or studied the beliefs and teachings of the earliest Christians?" Or, "Have you ever examined the claims of the Orthodox Church to be the living Church as described in the New Testament?"
My advice is to get a firm understanding of the early Church's situation, what they believed, what they taught, how they prayed and worshipped, etc. Find a way to tell this in a compelling way so that they can see why Orthodoxy makes the most sense out of the historical story of Christianity. Even have them try to do it. Say something like, "So, tell me the story of the Church, as you understand it, from Pentecost to today." Then you can tell the Orthodox story.
Chances are, many of them haven't studied it much and aren't aware of the Orthodox self-understanding. Giving them the true historical story of the Church will be the best thing you can do.