Heat!
A 'Mole' wrench
Heat!
A Stillson pipe wrench (one of those pipe fitter's things, with sharp teeth that grip the object that is to be turned) Or else a 'Footprint' wrench, same thing.
More heat!
Cut a slot in the plug that you can get a screwdriver into, from the side and use that as a lever. Probably best to add the Mole wrench across the cut to avoid breaking half off.
When half does break off, go back to the Mole wrench!
Whatever, try to tighten it first, before undoing it.
Heat is important because a ring, as in the threaded fitting welded into the sump, will expand more than a lump, as in the plug.
You can easily heat the thing by taking the car for a long enough run to warm up the engine thoroughly.
But if that's not possible, just use a butane torch, with appropraite precautions to prevent setting fire to thing in the engine bay. The oil inside will be an enormous heat sink, and there is no risk of boiling that or setting it on fire.
Unless you use a gas axe,set to cut!
or.....................
http://www.gelplane.co.uk/pelapumps/6000use.asp