Best bet is hammer and chisel.There's enough plug sticking out to use a chisel, give it a few bats from one side then a few from the other, trying to unscrew it anticlockwise. I've had quite a few to get out, but it,s not easy to get at.
HTH jon
PS if you get it out, get someone to weld a big nut onto it so you can get a spanner on next time