You can also try to put a little sand in the socket. I've used this many times working on trucks. Just enough to get a good grip. That is usually my first choice, then the pipe wrench or vise grips. As mentioned above if that does not work then weld a nut on. Just make sure you have a new one to replace this one with now. Also, once it's out, examine the threads because it may have been slightly cross threaded as it was put in.

I did EXACTLY this on my 260E's first oil change.

Rounded it a little; smaller, 6 point socket, rounded it more, vise grips (12"!!) rounded it more, pipe wrench fully round by now. I tried the sears bolt extractors over the outside. Drilled most of the way through & tried an EZ-out. I had visions of drilling through, letting all the oil out and then snapping an EZ-out in the drilled hole so it would never hold oil again. What a *****!! Finally, the chisel on the edge trick. I took a couple of curls of the plug material off and then it turned about 1 1/2 RCH and came out nice as pie. There wasn't a touch of corrosion anywhere. It was beautiful once you got past the remains of the hex. The plug and its copper washer had just frozen to the pan as tight as any two metals could.

Now I use a dipstick sucker outer and don't worry about it. suckers link

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Norm in NJ
Next oil change at 230,000miles