Hi. I am in Year 8 and have secured a Grade 5 in writing. I would strongly recommend you to include 5 verb tenses:
He hablado - I have spoken
Voy a hablar - I am going to speak (informal future)
me gustaria hablar - I would like to talk (conditional)
Hablé - I spoke (preterite)
Hablo - I speak (present)
Make sure that you use comparatives eg. something is more better than something else.
For example: las verduras son mas saludables que los churros (Vegetables are healthier than churros)
Use decent opinion phrases: Me gusta mucho las verduras porque, en mi opinion las encuentro deliciosas
Using more complicated opinion phrases can push you into a Grade 6, especially if you include another verb tense in one sentence but for a Grade 4-5, I think that this should be enough.
Revise through the verb conjugations for the different forms of people as including another form other than the I form will increase your chances of a higher grade. From personal experience, an easier form to include would be the we, considering most its conjugations end with mos meaning its easier to remember. I am trying to push through to a Grade 6 now on the AQA Examination Board and recently discovered that using the subjunctive can push you into your grade 8s if you have excelled in all other aspects of your writing. Hoping that this helps.
If anyone else needs any help on how to obtain a Grade 4 or 5 on the AQA Examination Board, I'd be happy to help.