Just need a 550 on GMAT. What's the most efficient way to study?

Hello all, just like the title states, im only aiming for a 550 on the GMAT. I was wondering what's the most efficient way to study, and which materials to use?
I really don't want to have to practice and go through 650+ level questions. Not trying to be lazy or rude, but i dont want to waste my time practicing or going over 650+ level questions.

haha, please skip the next 2 paragraphs, they're just about my background and me ranting

I started studying in september 2014, but have been on and off. Started studying more and more often in Jan of this year. Started using gmatclub to get extra questions. so far i've just been answering sub 600 level questions, getting mixed results (i feel like its partly due to some of the questions there aren't actually 550 level questions)

So far i've gone over foundations of GMAT math and verbal and most of the guides in manhattan. im also answering questions in the OG guide. i can see my vebal has definitely improved, especially sentence correction, which makes me happy :D
but im still having a little trouble with the quant... part of my problem seems to be careless mistakes, overlooking a certain detail, small things, im fine with those. what i really hate is, that ive reviewed pretty much all the manhattan and of guides for quant, i GET AND UNDERSTAND all the explanations and examples, but it seems like actual gmat/practice questions are in a way different league... even some of the 550-600level questions

i plan to take the actual GMAT sometimes in april (havent signed up for it yet). im going to be taking my first practice CAT tomorrow from kaplan and am aiming to take at least 1 or 2 practice cats a week until my test so i can get used to the situation.
so i was wondering if you guys have any tips for someone who's just aiming for a 550. my main problem is, how can i possible memorize all the formulas for quant? i have flash cards, but i feel like theres so much info, even if i memorize it today and can apply it to problems, by next week ill completely forget how to do it. i just feel overwhelmed that theres so many concepts to memorize in quant...