Removing greasy nut

Trying to remove the cover under the fuel filter on my 91 300ce. Got all of them off except for one which will not budge. I'm using a 10mm socket which is the right size (9 is too small) but the socket just
slips when I turn it counterclockwise. I cleaned the grease off but it's still slipping. I REALLY do not want to take a drill down there, and the space is too tight to get a vise grip in there. Any ideas?
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Get bolt out from Sears... they are onsale now... will grip onto the head and remove it. It fit onto a socket wrench.
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