Don’t overrate a native speaker of English who live in Japan. They are almost dropout of mother-countries. They can’t show you clear and sharp aesthetic English. They can correct some words spelling and some basic grammatical mistakes at the most. But wrong composition is wrong. Crows are never the whiter for washing themselves.
Good teacher is able to rewrite your awkward English composition, keeping essence of your outlook and intention, and could show you the example of clear and sharp aesthetic English sentences, like reconstruct the wrong design building.
Most effective way to improve English writing is practice. Next is reading great numbers of English books.