Please do not make such sweeping statements as this: Native speakers are not a good choice either since most of them have only marginal knowledge of what
is required. I know that this is hard to belive , but some of us 'native speakers' DO have more than a marginal knowledge of what is required, belive it or not, some of us also have an idea of how to teach.
..There are loads of people (myself included) who make their living preparing high level students for exams such as TOEFL, CAE, CPE, etc. - This is interesting, as seconds before you intimated that most native speakers have only a marginal knowledge - but you, of course, are an expert. I wonder why?