Late-ish response, but the exam hasn’t already happened at least and I’m having the same problem. I usually score very high on any type of English-related multiple choice, but between the practice passages our teacher gives us and the ones I’ve done on my own, things aren’t going so well. I bought the 5 Steps to a 5-500 Multiple Choice Questions practice book (which might help you, if you’re interested/don’t already have it, it’s on Amazon and at Barnes & Noble, and I’m sure other places as well) a while back and I’ve just recently started using it, but so far my results are less than ideal.
I know that building vocabulary and memorizing syntactical terms is the recommended study material, but where do you even begin? There’s just SO much stuff to cover, and it can be especially disheartening when you have NO idea what exactly you’ll need, so it’s basically just a shot in the dark. There are a few interesting PDF’s on the web (I think they’re from like, private schools for geniuses) that have whole lists of the syntactical terms their students are supposed to know by the time they show up to English class. I’ve copied and pasted some into a Google Doc for future reference, I’ve also found that the AP Lng/Cmp Crash Course study guide from REA (I think) contains a fair amount of helpful information (it is by FAR the most thorough one from that series that I’ve looked through), so maybe check that stuff out, if it sounds like it could be beneficial to your study process.
Anyway, I guess I’d also recommend reading a few books on the side (that’s what I’ve been doing)–it can never hurt to be well-read. Don’t just skim, close-read; pay attention to the syntax and rhetorical devices and vocabulary–look up the words you don’t understand, etc. Who knows–maybe a passage will be on the exam! As for the approach of the multiple choice, we’ve all heard the method of reading the questions before the passage, but I have always felt that colored my perception of a passage and didn’t allow me to form my own thoughts on it–maybe that’s just me, though.
I’m getting a little nervous at this point–I knew this would be difficult, but I didn’t realize just how hard it is to get a 5 on this exam! (I’m sure there are harder ones like physics and chemistry, but I’m definitely not taking those). By the way, how are you feeling about the easy portion of the exam? I’m thinking it won’t be terrible, but then again, timed writes are hard, especially when you have to write 3 essays! Anyway, good luck to ya! I’m sure you’ll do fine. If you ever want to discuss the class and/or commiserate, feel free to message me 
Also, if anyone else has any recommendations/advice, it’d be much appreciated–especially if you’ve already taken the exam and we’re able to achieve a high score with a specific study regimen. Thanks!