Grammar is an important part of the English language so you must try to show your examiner that you have some understanding of the range of English grammar. This could include using a range of different tenses, various sentence structures (simple, compound, complex). A good area to focus on is using a range of tenses – not just past / present / future but things like “perfect , continuous and modal forms”.
Don’t worry too much about pronunciation – the most important thing is that the examiner can understand the wonderful language that you are using. It takes years to master native speaker pronunciation in any language so your examiner is not expecting you to use “American or British pronunciation”, a little work on areas like sentence stress or intonation can help to improve your score.