I've gone and done it again

I was changing the plugs in a 500SEL and broke one off. This is the rear most plug on the drivers side so it's not the worst to get at its damn close.

I put platinum plugs in the car just over a 100,000 miles ago so as you can imagine they are welded in place. I sweated and cursed my way through 7 of them and have a blister on my hand to prove it

The last one went and broke on me so thats where I am now!

I've punched out the center so I have a nice round hole in the center but the only easy outs I could find are the ones that "twist" in and they seem to be causing pressure on the sides so no matter what I do I can't seem to get it out.

I've broken two sockets, the ones I put on the easy out and a 3/8's extension, to no avail!

Does anyone know of a "trick" or something that might help me out, I hate to think a "20 min. job" of changing the plugs could turn into the nightmare it has!!

Please help!!