You could try this, but I am not a tech or a pro, its juist my thoughts.
Liquid wrench or wd-40 let it sit on it for a while, then attack it again.
If all else fails, remember to oil the threads or use some anti seize on the threads next time.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.