What nut are you talking about?
There are 3 on the top that mount the strut top to the body.
There are 2 large ones that mount the strut bottom to the rear knuckle.
And there's one on the strut piston shaft that holds the spring on.
You remove the 3 top and 2 bottom bolts, then pull the strut/spring assembly out in one piece.
You then use a hex key or shock absorber socket (depending on the type of strut) to hold the piston shaft in place while you use a wrench to loosen the top nut.
You should use spring compressors. You can get a free rental (with deposit) from AutoZone, or buy a set for about $10-15 at Harbour Freight.
If you're talking about having a hard time with the single bolt that holds the spring on, look at the top of the piston. It will either have a hex keyhole for you to put a hex bit or allen wrench in to hold the shaft still, or it will have 2 flats on either side at the top. That uses a special shock absorber socket at fits over the top of the shaft and that's what holds the shaft in place.
This is what a shock absorber socket looks like:
If you've screwed up the top of the strut where the socket or hex key won't work, you'll have to cut the nut off. I don't think you'll have clearance for a nut splitter, so you'll probably have to use a dremel or angle grinder.
