Mine has always unscrewed OK with a large screwdriver. It maybe that someone has used locktite on the threads in fear of the drain plug coming loose. I would get hold of a large screwdriver and grind the blade to achieve a good fit in the slot. make sure that the shaft of the driver has a hexagon section that takes a spanner. Hold in place with lots of upward pressure whilst a second person shocks the spanner end with a hammer. Sometimes a shock in a clockwise direction first will help to break the seal.
The plug sits on a compressible washer that should be replaced, but I have only replaced mine once and have drained the leg 8 times.