Ask any JW if the Catholic church's practice of voluntarily confessing to one priest is unscriptural.
They'll automatically want to say 'yes.'
Then ask them why J.W.'s are compelled to confess their serious sins to three elders? (scriptural backing?)
Matthew 18:15-17 says nothing about elders being involved when someone commits a serious sin.
In fact, Jesus explicitly stated that if a serious sin has been witnessed by someone, the matter should be resolved by the sinner and the witness. No one else was to be involved. Jesus never said anything about Christians having to confess to elders. (verse 16 is referring to other witnesses of the sin - not witnesses of the accusation)
Get them to read Matthew 18:17 & 1 Timothy 5:20, and then ask them, "So exactly who did Jesus & Paul say were to be informed of an unrepentant person's sin?" (obviously not just the kangaroo court!)