There have been a few suggestions on LA/OC mechanics in the past year - the search function should yield good results
Off the top of my head,
Marx Mercedes is highly regarded, though pricier than most
DDK Motors also good
Mercedes by Maurice - though I don't have first-hand experience of these guys, I think ownership has changed over the years
Main issue (at least when I was last in the area, summer of last year) is that everyone is slammed at the moment, be prepared to wait for an available slot, and a reluctance to keep your car in the shop for longer than is absolutely necessary!
Good luck