I cannot offer too much advice for Critical Reading since it was always my lowest score and hung around the high 600- low 700 level, but I can tell you that for the passage-based portion, do not over-analyze! Unless the question explicitly asks you to infer something, all the answers will be in the passage and not require you to make your own conclusions. Essentially, any conclusions you DO draw should be backed with the passage text and you should most definitely never over analyze at all. As for the sentence completion portion, just memorize vocab and how its used, in addition to reading some books.
As for writing, however, I was able to get an 800 using a couple of books. First is Gruber's preparation for the SAT (something like that, you can check my other post, it has a link) and also the blue book (CollegeBoard official book.) Gruber's will give you problems that are of much greater difficulty than the actual SAT. The blue book will show you the patterns that are almost identically used on the real SAT. Basically, Grubers' hard problems will make the real SAT writing problems seem easier, and the blue book will present certain patterns through its problems that the real SAT uses, too. But, a word of caution, the blue book problems are a bit easier than the real SAT problems. Also, try to not get frustrated by Gruber's lol. I used to get SO mad that the amount of questions I missed would be so high when using Gruber's, but it really does help. If you have any questions, feel free to ask! And by the way, with that essay score, it should not be TOO hard for you to get an 800 on writing
. Good luck!
EDIT: And one more thing, it is definitely possible for you to score very high! My first time sitting I scored
WR: 690 (8 Essay)
And with a little change of mind and of course, more studying my second and final sitting scores were...
WR: 800 (10 Essay)
I did mess up my math a bit since I was scoring around 780 on my practice tests, but that just further shows how high you can score! IT is definitely realistic to raise your CR score by 100+ points in a short time and with some practice with grammar, etc., I am sure you can score and 800 on writing. Just remember, confidence is key!!