to undo the balljoint nut i'd remove the driveshaft nut,
disconnect the hub from strut,
full left lock to unhook the left driveshaft or full right lock to unhook the right shaft,
reattach the hub to strut with bolts but no need for nuts,
really clean n oil the rusty balljoint nut much as possible,
jack up from underneath the balljoint to lock the balljoint shaft into the hub using the cars weight to prevent it simply spinning,
gently loosen the nut with preferably a 6-side socket & breaker bar.
whenever it starts to stiffen, back off, clean n re-oil brefore trying again.
then seperate the balljoint. don't use them wedge tool that u hammer in cos they wreck the rubber boots unless ur gonna bin the arm anyway.