From your pic, it kind of looks like you are tightening, rather than loosening it. It unscrews counter-clockwise. If that's the direction you are going, and it's not coming, I suggest pointing a heat gun on low at it for about 7 minutes, and then the screwdriver and hammer.
If the nut is really that stuck, then the plate behind it will probably be stuck too, requiring heat again and little pry bars to get under it fashioned from bent-tip screwdrivers.