Witnessing, telling a lost soul the truth

Ok, I can understand atleast a little bit on witnessing to people who say they're not Christians. Once they find out the Bible's true truth will be easy to see for them. For, than they'll know the deity of Christ, His bodily resurection, grace and not works saves. Of course they need the Father to draw them to His Son, the Holy Spirit's conviction and saved by Christ alone.

But, here's the problem. I know people who think they are Christians with their own corrupted bible and other literature. How can a non-Christian who thinks he/she is a Christian and knows the truth realize the doctrines they're taught are from twisted scripture and unneeded literature? I know the Holy Spirit is the one to convict them but I need to know what God wants me to do.

They deny:

Christ on the cross

His deity

bodily resurection

saved by works

Jesus isn't the mediator for the majority of them

If something is said they can just go back to their corupted bible and false literature for "proof". The religion uses threats, fear, deception. And, if they get suspicious they'll think they're being persecuted (that's why in the future I have to leave so I don't get caught putting stuff up about their religion online.) But, first I really need to know what to do. The elder is smart, if he gets suspicious that could lead to trouble.

Jesus can take care of the fears. But, what I need to know, and am stuck on is how to reach a lost soul without falling into one of their pit traps (deception, fear and threats). And,...I just really need to know what to do.

If I tell them a truth they can just go to one of their watchtowers and show that I'm "wrong".