This is super common.
Did you use a 12 pt socket?
My theory is that this will not happen with a 6 pt socket, but someone may prove me wrong.
You could try driving a 1/2 inch socket on there, which is just slightly smaller than a 13mm, or perhaps a 12mm 12 pt socket. Chisel can work. Grinding new 'faces' with a Dremel tool, and using a smaller wrench or socket is yet another trick. Otherwise, the 'external' socket-type extractors Haasman posted are probably going to be the ideal tool for this one. The drill-type extractors are risky here, since you are in it deep if the extractor breaks. Unless the head breaks off, I would avoid that route.
Typical fallbacks like Vise grips, pipe wrenches, and such do not work well here due to the angle.
The underlying problem is the steel bolt in the aluminum pan, and an extraordinarily long threaded section for a plug bolt. A little teflon tape will prevent this problem.
1986 300E 5-Speed 240k mi.