try belting a slightly smaller socket onto it, especially if you have already done a good job rounding it :) .. have you tried a little bit of heat with an oxy or a one of those bernzomatic butane burners? tried pouring penetrene on it liberally for a few nights prior to undoing it? also wurth do this spray that has penetrating oil combined with a freezing compound call rost-off which not many people know about but really helps. one of those metre long torsion bars really come in handy for doing jobs like this.