I see, I thought grades and essay make up one part and there were other parts. My bad.
I wish you well but I don't think there is much I could do to polish a really good essay into a perfect essay in a post or even several posts. Even if I saw your essay, this is close enough that I'm sure it is more about what exactly the admissions committe is specifically looking for in their candidates rather than anything universal.
There is the basics, of course, make very sure you have everything spelled correctly (read it word by word from the end to the beginning to help avoid seeing what you meant to say rather than what you actually said). Either try to find some naturally good spellers who would read it over for you or spend a few hours with a dictionary and/or English usage website, looking every up everything you are remotely uncertain about. Same for grammar
and punctuation. And write it early so you can finish it, then set it aside for a few days or weeks and then go back to it.