The good thing is that the Superintendent won't be shocked by your statements; as you note, he or she already is aware that multiple people have problems with your boss.
If it were me, I would start out by thanking the Superintendent for taking the meeting and state that I have some concerns that I have been seeking the proper outlet to share and would like to speak in confidence. Then I would diplomatically air my concerns and ask for any constructive ways to deal with the issues and personalities.
Depending on the reception I would gauge whether to continue on to specifics of the case being built against me...there may not be a lot you can do. It can be frustrating how long these type of people are allowed to go right along their merry ways. Good luck.