From my experience, Spanish writing at both GCSE and at A-Level depends greatly on the quality of your essay. It is divided up into 4 marking sections.
Content of the essay (/25)
Range of vocabulary/phrases
Complexity of vocabulary/phrases
Accuracy
In the content section, it is essential to have a thorough understanding and knowledge of the task, a wide range of relevant examples and evidence, clear evidence of evaluation and well-justified personal reaction and a well-organised structure with clear progression. If this is in place, the rest of the essay should flow nicely.
Three top tips from me:
Use as many different verb tenses as possible, but make it sound natural and ensure it is accurate
MAKE A PLAN (for 5 mins) before you begin writing the essay
In your conclusion, finish off the essay with a quirky, memorable judgement