Hey, I'm Eiken 1kyu and TOEIC 990 holder, but I can't write "perfectly-grammatical-without-any-errors-textbooklike" passage. In fact, my score sheet of EIKEN said, "Content/Logic 100% / Grammar 70%", I don't remember exactly, but it was like that. If your "content" and "logic" is perfect, why should you care about GRAMMAR?
That's so true. When you're able to produce perfectly logical meaningful sentences, then they can be easily make into grammatical ones with some translation tools, which are many out there in the Net. Also you can find some people who're specialized with writing proper material with required language in the fields, so you can ask them to rewrite yours into more sophisticated ones. Grammar is there to make our communication easy, right?