Things you will be assessed on:
In terms of tenses, I would recommend using plain form - makes it shorter, simpler, and illustrates a higher level of Japanese. If not, polite form is acceptable. Make sure you use one form throughout the essay to illustrate consistency/coherency.
Before beginning the essay, I strongly advise making a plan, so that you can follow this structure throughout the essay.
Structure:
Introduction: This largely depends on what question you are given and what topic/section you choose to answer. Reword the question; clearly state what you will be discussing/your argument so that this can be developed throughout your answer. In the case of a text i.e. 'Duke' by Kaori Ekuni -you would perhaps include quotations from the book and key scenes you will mention, and what you will discuss in your essay.
Main body - Discussion of the question. Ensure that you have a clear, logical structure. Use the genko-youshi to your advantage. If it is helpful, use a Point, Evidence, Explain, Link (back to question) (PEEL) structure. Answer the question and do not include anything that you think would not get marked. Ensure that you have revised your topics thoroughly to be able to answer questions in depth. Make sure you try and use a range of verb forms, grammar, connectives, relevant vocabulary and kanji.
Conclusion - reiterate your points and summarise the important points made in the essay.
I would be willing to give my past essays to illustrate how I answered a various amount of questions covering a wide range of topic areas.
Notes: To revise essay writing for Japanese essay exam questions, I think that essay planning is the most effective method.