I can't recommend a coach, but there is one thing I taught a friend which basically got her to 90% porteño accent: the aspirated S. Most instructors teach you the obvious stuff (like the "yeísmo"), but no one seems to touch on this topic. There really is a before and after in how porteño your accent sounds once you start doing it.
Basically, you need to make an aspirated S (which sounds like a slight English H) every time there is a consonant after the S. A porteño would never say "España," they will say "Ehpaña." "Estar" becomes "ehtar," "Pascuas" is "Pahcuas," you get the point.
This also applies to an S at the end of the word if the first letter of the next word is a consonant, e.g. "los perros" becomes "loh perros." So you would say "¿Ves?" if that's the whole sentence, but you would say "¿Veh por qué?"
If your level is already C2, with this and the yeísmo you'll be 90% there. The rest is, as others have mentioned, tone, pace, and slang/vocabulary, but if you're a C2 you probably have a good grasp of that already.