Hola Daniel. Welcome to italki ! Can I ask which exam are you preparing for? (IELTS, TOEFL, Cambridge, etc.) and what grade/level/band score are you aiming to achieve? As you are concentrating on writing, I would suggest researching professional teachers who have experience teaching exam strategy for the particular exam you are preparing to take. I specialise in Business English and in exam strategy for the Writing Part of the the IELTS exam (both Academic and General). You could also explore the Italki community pages a bit further and post examples of your writing for native English speakers to correct for free. If you have any further questions, please message me.