If you want to pass an official exam, you want to demonstrate what you know. Although you may never use some of those words, expressions or phrasal verbs again, you need to take the opportunity to use them on an exam. If you don’t use it, we assume you don’t know it. Also, don’t be afraid to be bold and take risks. Often times it’s better to attempt to use an expression and not use it 100% correctly than to not try to use it in the first place.
While some expressions are difficult to learn because they don’t seem to making any sense, there are others which are almost exactly the same in Spanish and English. I would encourage you to learn these because you are much more likely to remember them because of their similarity to your language. Consider the following ones and see if you can think of their Spanish equivalents:
I’m between a rock and a hard place.
Live and let live.
Kill two birds with one stone.
It’s impossible to learn all the expression there are. Pick several that you either like or you think you’ll remember easily and learn those by heart. If you use one or two while writing or speaking during an exam, that could be enough to earn you the points you need to pass the exam you’re taking.