If, as your name indicates, you are a young lady.
Just stand at the side of the vehicle and look all feminine and helpless.
( Simpering and speaking in a " little girl " voice usually works wonders ).
Failing this, LOOSEN the three movable nuts and then gently move the car back and forwards a few times.
This may free the offending nut.
As you pointed out, care must be taken not to " round off " the wheelnut, so, place the wheelbrace on the nut,
prop up the shoulder of the wheel brace,
( your jack is ideal for this as it can be adjusted for height,
and the idea is to keep the wheel brace as straight as possible ),
This enables you to use your full weight on the wheel brace without it slipping off and rounding the nut,
NB. A spider, or cross shaped brace is ideal for this job.
Failing all else, or if the nut is already rounded it can be destroyed useing a " Nut splitter ". But this is a garage job.
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Here's a tip, When you have a wheel fitted at a garage, ask them to put a smear of graphite grease on the threads and then tighten the nuts yourself.
Garages use an air driven wheel brace that jams the wheelnuts on so tight that us blokes find them hard to remove.
Application of heat and a liberal overnight dose of WD40. might also help. Just remember to wipe off any surplus WD40, or you will help the brace to slip.