Here's another Victorian poster!

Firstly, I just want to say that I totally agree with everything that has been said above. However, having identified that you feel that your biggest issue with phrasing is spelling and grammar, I would suggest that you do everything you can to improve in that specific area. I totally sympathise with you, it is hard to practice, but honestly, grammar is one of those things that is so underestimated yet so so important. From my own experience, I have found that if you can grasp and effectively utilise correct grammar mechanisms, your writing suddenly becomes much easier for the marker to read and understand. Ultimately, the more clear and concise your writing is, the easier it is to mark, and the easier it is to mark, the better the score you will get. Believe me, there is nothing quite as joyful to a literary fan, than reading a piece of wiring that is clear, concise and sophisticated!
In order to improve your grammar and phrasing, I would suggest a few things; firstly, spend your spare time reading good literature. None of those trashy tween fiction adventure novels, I mean proper sophisticated literature such as Charles Dickens. The classics are always a good place to start. Stretch yourself and remember that it doesn't matter if you don't understand the level of literature being used. Just take it one step at a time and look up words you don't understand, and soon you will begin to incorporate sophisticated phrases into your writing without even realising! Reading is so important because not only does it introduce you to new words and ideas, it also provides an exemplar of what good writing looks like and helps you identify the gap between your own writing and the authors writing.
On the spelling front, I would use a dictionary to spell every word you are not sure about. I don't know if you are allowed to use dictionaries in your assessments, but if you are, I would suggest that you take advantage of this privilege. It might take a bit of extra time, but it is well worth not losing those critical marks that make all the difference. Also, you could come up with little tricks to help you remember difficult words, for example when I was in primary school, I used to say the 'c' in scissors to help me remember it was there!
Lastly, practise, practise, practise! Even good writers have to constantly write in order to improve. And I don't mean that you should just write loads of practice essays. Start a journal, pen a creative short story, whatever it takes to get you enthused! After you have proofread your work, (reading it out loud is the best way), get a mentor, (preferably someone picky who will find grammatical errors and poor phrasing), to read over it and provide constructive criticism. After this, go back and use their suggestions to improve your work. This is extremely crucial, as not only will it help you to reinforce their feedback, but also reading the finished result will give you an amazing sense of satisfaction when you realise that you
can write a well structured and smooth piece of writing!
I hope that some of my ramblings make sense and help you to become an even better writer!

Good Luck!