How can I ensure I do well in my Oral or Writing when I do not know what the question or topics will be, I always get 'brain-freeze' under pressure?

Exam pressure and extreme stress stop our brains working effectively, so the best way to ace the oral and writing exams is to trick yourself into feeling confident and calm. This is actually easier to achieve that you think. What you should do is get some perfect sentences in every topic-area ready that you could recite asleep, so when you get to the speaking, you know you are going to score some brilliant points almost effortlessly. This will put you at ease, and allow you to be adventurous and show off your skills. You’ll soon see your brain will be on fire rather than freezing up!
For example, for the Family topic, let’s get a subjunctive ready, and some excellent vocab. You could have something like:
‘Me gustaria haber tenido un animal en casa, pero mi hermano los tiene alergia. Cuando vive sola, tendré un largartija, porque me interesan los reptiles.'