That one looks pretty bad but it will still shift, clean off the visible threads with a wire brush first & if you haven't got access to a proper plug remover attach a big set of mole grips across the best looking flats that still allow you to apply force. Then first try to tighten the plug, I know that sounds counter intuitive but the very slightest of movement is all you need & it will then unscrew. Many moons ago I picked up a purpose made spanner that has never let me down .....probably fatal last words!