I'd heat the plug with a propane torch for about three minutes. Then I'd allow it to fully cool. Then, I'd use either PB Blaster or Kroil on the plug and allow it to fully penetrate for 24 hours.

Then, find a six point box wrench and give the wrench a sharp hit with a hammer. Either you'll break it loose instantly...........or you'll fully strip the flats..........but, I'm betting on the former.