You shouldn't worry too much because many people who get A's in French only get half marks in the listening. How well do you do in the reading and the 2 writings? Because if you do well in those then you can afford to lose marks in the listening.
However if you want to improve your listening, coming from someone who got 20/20 in the prelim...
You need to be able to really concentrate and block out everything else around you, just focus on the French and make sure you can pick up as much as possible. I find that when I am less focused during a practice listening in class, perhaps i am tired, then I am unable to pick everything up.
In order to hear the words you don't know and write them down to look then up in the dictionary I would certainly recommend trying something called dictation. Basically it is where you listen to French and try to write down everything that you hear accurately in French not English, correct spelling and correct accents. Our teacher has done it with us a few times but if you search around online I'm sure you can find exercises that will do the same thing.
Additionally, although it might be a bit late now, if your knowledge of vocab is not good you should probably improve it...
And if you have no clue what something means just write an educated guess based on the question...
If I think of any more advice I will let you know!
Even listening to French music might help. A friend of mine (who also does very well in listening) introduced me to a French cabaret singer from the 1950's called Edith piaf
I would not aim for 20/20 btw very few people do that well, very few people even get over 17. So getting anywhere from 12-15 I would say is a perfectly good mark