Arguing never work because that's what jw's love to do. Core doctrines ? The absolute core is the Governing Body and thinking they are "special" and are the "sole channel of communication between man and God" that is the core. After that delusion is dismantled, all doctrines are up to question , since the GB is NOT who they say they are. Then thier peculiar doctrines making them the "central stars" of the Bible, becomes clearly nothing but embarrasing grandiosity.
Lots of churches believe in a 1000 year earthly reign of Christ and all different kinds of "bodies" physical, and "spiritualized physical" and variation on the theme is out there. The JW doctrines are NOT unique !!
Thats what he thinks, that he has "Special Knowledge " This is the core. Special Knowledge. Exclusive knowledge. " I know something you do not understand" That makes me "special" I belong to a "special group" of exclusive select who follow the only channel to God. That is what he believes. Not the way he would say it, but he can't say it out loud, because its too ridiculous.
The first thing a JW has to do to is clear themsleves of this core "special knowledge" idea. There is no special knowledge, everything knowlable, can be known, by anybody, without attachment to special guides or allegiance to ritual or mysterious groups. The core book to unravel his delusions is still Crisis of Conscience, written in the Watchtower Style. Thats why it works.
After that, there is the long step down - off the high horse - to being just one of us regular confused slobs who accept and respect other people and their opinions and ways of worshiping as just as legitimate as anything we or anyone can think up. Its a long step, many never make.