I don't think it is a left-hand thread - just a nut that unscrews in the normal way. I presume you have the wiper arm off already. The large outside nut should really not be all that tight. There are two Philips screws to remove - unfortunately, those often seize in place, so use a screwdriver that fits really well. Maybe a large screwdriver bit and a spanner to give best leverage.
-Alex