How can I reach the top grade band in my writing section?

I would say there are three key ways to achieve this. Firstly, it is hugely important to use an advanced conditional phrase which involves the imperfect subjunctive. For example, if you were writing about the environment, you might say 'si recicláramos más, habría menos polución'. This is an essential structure if you want to be getting top marks. Secondly, it is important to begin your paragraphs well so the examiner gets a good impression from the get-go. There are various ways of doing this, from getting a subjunctive in immediately (e.g. no parece que haya...) to using impressive connective phrases such as por lo tanto, sin embargo, or no obstante. Thirdly, to put the icing on the cake, throw in an idiom or two to show that you know more than just how to translate your English thoughts into Spanish. A good example that can be used in most contexts is 'al fin y al cabo', which means 'at the end of the day' or 'after all'. Make yourself a mental checklist of these before the exam and tick them off as you go.