How can I elevate my language for both the oral and written exams?

Of course grammar and vocabulary is very important but I think that by using idioms and phrases it is easier to achieve apparent fluency in a language. Many elements of the oral and written sections involve debating and arguing a case. Therefore having a number of very general phrases in your backpocket automatically increase your language level and will look very impressive to an examiner. Such idioms are: a mio parere/a mio avviso.../ e' risaputo che.../ or even something like "il rovescio della medaglia" (which literally translates to "the other side of the coin", but is used more to mean "the downside").