+1 on the impact gun - I had this same issue when installing Bilstein damptronics on my PASM-equipped Cayman. You will have to drill an elongated hole in the impact socket that will allow the PASM wire through the slot and then you can begin to loosen the nut. Take it easy with the impact gun, use quick, brief "blips" of power to get the nut loose and not damage the wire.
I used a regular pipe wrench on the washer. For the nut there is a proper socket for it (Schwaben 003046SCH01A-03) but as mentioned you can modify a socket or just use an open wrench if you're not fussed about torqueing to spec.