I had one like that too. It appeared to be fused on. I even resorted to using a pry bar. After it finally came off i couldn’t be sure it wasn’t bent.
Simple solution, this is a common MB Engine. I went to a local pick and pull. Located a donor car and bought the valve covers for both sides
I paid $40 for both

The breather came off the donor covers in less than a minute. You might want to look into this option
fast, easy and cheap