First soak the bejesus out of it with some PB blaster and keep it wet for a day. Then a small propane torch would help heating the top nut some. Then weld a scrap piece of iron to the rod part of the end link so you can hold it in place while you bust the top nut off. Since it’s breaking wrenches.
If that doesn’t work, then as @TomDirt suggested, take the sway bar off dropping it down, giving you room to work. Grab a cutoff wheel and go to work on that end link.
