Remove Fastener Nut for Kitchen Faucet (Rusted?)
You might find a monobloc tap wrench that fits. They are designed for reaching up behind a basin.



Or a conventional box-wrench set, though I'd guess they are usually not long enough. You can use a normal 1/2" drive socket set to extend one.




If you are replacing the tap, you could use a rotary tool (e.g. Dremel) or similar to cut off the bottom part of the threaded tube to gain access to the nut at the top without interference from the connector at the bottom.




  the nut that is hold that brass plate has corroded.


Clearly, water has been leaking through. You might just have limescale deposits that can be cleaned off with a limescale remover and perhaps with a wire brush (perhaps using a rotary tool again)