I'm hoping you have it out by now but here is what I would do. It looks like from the camera angle you can do this. Spray it with penetrating oil. Get hacksaw or sawsall blade and cut a nice slot in it only as deep as the hex on the plug. Don't cut into tranny housing, just do a gentle cut. Before cutting get a punch and a hammer and put it in the center of the plug and rap on it a few times just to get a "shock wave" through the plug and this can loosen any bond the threads are having with the case. Gently file all sides of the hex head smooth being very careful that the filing is square to the case and the other sides of the hex head. Get your best flat screwdriver, nice and square( clean it up on your bench grinder if you have to) and put it in the slot you created. Give it a slight tap so that it is seated while still holding it with you hand. Now grab the knipex or channel lock pliers with the other hand and turn both the screw and pliers at the same time. Good luck!