I have not had to fight an assessment,
however, I would think that the basic for your complaint would be the Comparables in the neighborhood:
I would take a clipboard and go door-to-door and interview at least 20 or 30 and preferrably 50
write their address, last sold date, selling price and their assessment.
You may be able to get a lot of information from your Assessor's Office; they may co-operate.
Good luck and may God bless