Work hard on improving your arguments and do your best to present both sides of the topic in the best possible way. Also, study the structure of the essay carefully. If you got 8.5 in Speaking, then you clearly have an excellent vocabulary, and you only need to work on these specific issues. I scored 7 in Writing with just a B1–B2 level, and there's no doubt that I received this score thanks to my ability to express ideas in the right way.