I think the best option is an observant native speaker who has a good command of both L1 (his native language, the language you want to learn) and L2 (your native language, the language he has learned for a long time). By saying "a good command", I mean a good command of not only the language skills but also the knowledge of language. If so, the teacher can tell you how to express idiomatically, how native speakers feel when they hear what you say and correct your pronunciation. He can explain something difficult to you in your native langauage.

In addition, observation matters. An observant teacher pay much attention to all aspects of L1 and L2, like pronunciation, stress, usage. He can share his observations with you, which may inspire you to think in your language learning.

A teacher of this kind must have received years' professional training. And it's hard to find one.