If you are prepared, the chances of getting a totally unfamiliar topic are very low. The best way to make sure you are not surprised by the topic is to do lots of reading in English. Read English magazines, books, news articles etc. This will make sure that you are familiar with a broad range of vocabulary.
Another great way to prepare for the writing section is by getting professional feedback – we offer this service at AEHelp. Click the “task 1 and task 2 buttons in your my student account to access this feature!