I finally managed to get it loose !!
I sprayed the bolt with penetrating spray after I used my Dremeltool to repair the bolthead.
I gave the spray a few days time to do it's work spraying again every day.
After that I used a new 6-point socket ( Thanks for that pointer Michael ) and a BIG hammer
I thought either it will come loose or I'll break it off.The bolt decided to come loose.
I allready bought a new bolt for the freaking price of almost 13 Euro.
MB had to order it from Köln-Germany cause it wasn't an normal stock item.
Anyway I'm very glad to have this problem solved and now I can get on with putting on new callipers and rotors.

THANKS EVERYONE FOR THE GREAT INPUT.
This board is proving its worth time and again.