I've got A* in all french mock exams and all the speaking/writing exams I have done so far, the key in the writing is to minimise grammar mistakes, use several verb tenses and advanced or irregular verbs and avoid repeating the same word or phrase (use car instead of parce que, etc). In one section, I would use some very advanced French above gcse level just go get a few extra marks. In the speaking, learn a piece, and again, speak fluently and with a nice accent, and again use several verb tenses and irregular verbs and avoid repeating the same word or phrase. For the exams, do a lot of past papers so you can learn the words that keep recurring and also get better at listening in general
hope that helps