The discursive essay contains several hoops that you have to jump into if you want to achieve a successful mark. You can gain or lose a lot of marks simply for your structuring of this essay. The best things to do to improve the structure are: 1) Include an introduction, briefly explaining some context and hinting at your own standpoint on the discussion. 2) Start with opposing arguments (ie arguments that you do not agree with) and write a short paragraph explaining some of these standpoints. 3) After this, write at least two long paragraphs, rebutting the opposing arguments and creating new arguments for your side. 4) Finally, write a short conclusion, summing up very briefly what you have said, and perhaps adding in a further argument for your side (if you are still within the word count).
The key thing to remember is to back all of your points up with evidence. This evidence does not necessarily have to be completely correct but it makes it look like you have researched this subject and gives you authority in your arguments.
Finally, it helped me to keep a list of complex grammatical structures that I wanted to use in my essays and check them off when I had used them - this will encourage you to improve the quality of your essays and will let the examiner know that, in addition to you knowing all about the subject of the question, you also know how to work with complex grammar.