Thats a tough one. I think first you should buy a new drain bolt.
Then I think you need some liquid wrench or some penetraiting oil to try to help loosen that sucker up.
Perhaps your copper washer is fused to the bolt and oil pan.
You could try heat, but I wouldn't recommend putting fire that close to the engine or oil pan, you might damage the pan if you use heat.
Have you tried vise grips with a piece of pipe on the handle?
If you need more torque, and can get a wrench to grab the bolt well, you can add a piece of pipe over the wrench handle to generate more torque to aid in getting it off. a similar principle to extending the lug wrench handle with a piece of pipe to make it easier to remove lug nuts on your wheels.
I'm not a tech, but I'm hoping that will help you.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.