Have a welder around? Or a buddy that welds? Weld a nut to it, the heat from the welding will help break it loose then use the new nut to take out, Would suggest doing this pan off (trans fluid is flammable), yes you would make a mess and have to refill. But if you pull most of the pan bolts (keep a couple in on a edge, but loosen them) then let the pan down on a angle to drain most of it out before pulling the whole pan. Have used this technique quite a few times (but not on a pan with trans fluid present).

 

Thanks Guys, and believe me I will try all that I haven't tried already, and am looking for that socket set that cuts in to the bolt head, but the head of this plug is really soft metal and it might just chew it up more !! really I am just waiting for my new plug to arrive, thanks again All , Cowboy