Hi, am a polyglot who speaks 5 languages. I did Russian GCSE in Year 10(I'm a native speaker), was getting 9s in GCSE French mocks since the beginning of Year 10 and get 9s in GCSE Mandarin now(I'm in Year 11 now). I am also aiming for a 9 English Language GCSE despite I only moved to the UK less than 3 years ago.
1)First, forget what your teacher advised you because there is a huge chance it was aimed at students who just want to pass. My teacher advised us to use Duolingo. If you want to get any more than a 5 just don't do that.
2)Print the specification and learn the vocabulary inside out. In your exam there will bot be much more than what's written there and 75% of your GCSE is just knowing the vocab.
3)Be fluent with basis grammar - how to use present, future and past with no mistakes. Memorise some sentences with more advanced vocabulary like conditional tense.
4)I don't know how much it applies to the languages you learn but you have to know the reading rules. If you don't, you will memorise the pronunciation wrong and even if examiner will understand what you say it will cost you marks.
5)Read the speaking marking scheme, you may be very surprised. For example, if you do AQA your photocard is only marked on the communication which means you won't be marked down if you do any grammatical mistakes.
6)For writing I recommend to write some essays and memorise them. I also used to think you need to use lots of fancy vocab but you really only need to use "range" of vocabulary, so there needs to be some variety. Once I didn't prepare for my French writing mock and ended up getting a 9 just writing about my last weekend. It just needs to be clear and have as little grammatical mistakes as possible.
7)You can write the same thing in both of your writing questions as they are marked by different people.
8)Buy workbooks or print the worksheets for your exam board to make sure you know all the topics well and identify the weaknesses.
This is everything I can think of straight away, but if you have any particular questions I will be happy to answer them.
Good luck!