It's my weakest too
I do a lot of past papers, making use of the first 5 minutes you get to look through the paper to jot down any words that will probably mentioned to answer the paper. For example there may be people talking about what they're doing in a day, and you have to match up the pictures, one may be a shopping picture, but you must note if it can be said in different ways so you don't miss it. For example, going shopping can be wrote as 'faire du shopping' or 'aller au magasins' so make sure to listen out for that.
Unlike maths or science, listening to the French language isn't something you can learn over night, it's a developing skill and with a month until the exam it's prime time to start doing so. Do this by listening to French music, watching French shows (there's a show called petit potam on youtube which is a kiddies cartoon about a hippo - slightly boring but the vocab is relaxed so you will understand more) there's also some French shows in the international part on Sky TV, also just speak French out loud yourself so you can familiarise yourself with the accent and the pronunciation of words.
For the exam itself, I'm unsure of how helpful this will be but it's worked for me:
I used to tend to try and focus on what every word means, but it meant I ended up missing the second half of the sentence and before I knew it I was just guessing the answer
Now I listen to the phrase as a whole, thinking in French as I do so, and then the second time round I listen out for any key parts which I may have missed, I find this to be a lot easier
I also think about how the words said would be spelled - this probably is only me but it helps me visualise what is being said and henceforth understand it better