Soak with penetrating oil, loosen the nut a few turns, leaving the nut on. Raise the weight of the car and support under the nut on a solid piece of timber or a concrete block or similar. Lower the weight onto the nut. Heat around the joint and a good whack or two with a lump hammer usually frees it. Sometimes you have to jack up high and hit the nut upwards with the hammer, taking care not to damage the thread in the process. It's a brute force job when they are really stuck/rusted. Take care under the jacked up car and support as necessary to avoid getting crushed! Best of luck with it.
Some light reading:-
https://the75andztclub.co.uk/forum/s...oving+wishbone