As good as this method can be I don't think it will help in this instance.I am going to try the heat and wax first though.
Profnohair
A sump plug has a soft washer which seals the plug to seal the oil in the sump so will stop the wax from entering the threads. and is the friction on this washer that stops the plug from coming undone.
The first stage I use for this type of repair is to give the head of the plug a few taps with a light hammer directly on top which some times compresses the washer which relieves the pressure so may be undone with whatever you can grip it with.
Don't hit through the shot with the hammer which may dent the sump if too hard but hit and and bring the hammer away as if you were trying to kill a wasp on the window without breaking the glass.
If after a few attempts this fails then as has already been sugested ,welding a large nut onto the plug.
good luck!
dave
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