A few years ago I had a similar problem. I ended up rounding off the flats of the plug head. I finally got the plug out by using a screw extractor or "easy out".
You drill a (concentric) hole into the plug and screw the extractor (which has a left hand thread) into the hole. Continue turning and the plug should come out.
The plug is approx 30mm long (Including the head) so there's plenty of material for you to drill into.
I understand that the drain plugs are a soft magnetic material.
Good Luck

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1990 300CE-24 (Gone but not forgotten)
2002 SLK 32 AMG
2002 ML320 (Gone)
1995 E320 Cabriolet