Firstly, read the question carefully and understand it completely. Secondly, answer the both sides of the argument. Only justifying a particular aspect of the argument will not be received well in terms of marks.
Speak calmly and with authority. There is no need to panic at the time of speaking as that will increase the chances of mistakes. Also, try to avoid the repetition of phrases as overdoing it will hurt your marks in the exam.
Practice, Practice, Practice. Practice hard and regularly and you will be surprised as how easy the test will be on the test day. Practice is what will make you confident in achieving your desired score in the exam.