battery stuck in its housing

you have to take more than the cables off. Some batteries are secured on the bottom, with a little bracket, that can not be seen easily from the top, just because you don't see an obvious tie down on top does not mean it's not still bolted down

expect a lot of disassembly if that doesnt work

to second davids answer, every car has to have a battery hold down clamp or strap (federal law)...Merc's is usually held in by a 10 millimeter head. the clk should be a very small metal clip on the narrow end of the battery. Staring from the front of the car look to the right side, you will need a long extension to get to it...let me know if it helped