chrisyip - I was coincidentally in the same position. From an engineering background, rocked the verbal and struggled with quant. I had the same time issues with the tests. Very frustrating.
Unfortunately I didn't get above 80% (I got a 46 - 75%), but I did get above a 700 thanks to my verbal (41), so laythesmack23 is right about being able to get above 700 by rocking the verbal.
What I found helped me the most with quant was flash cards. Practicing problems is important, but actually learning the formulas and the rules was important as well. Every time I found something that I didn't know right off the bat or something that the
MGMAT book recommended that I memorize, I put it on a flash card. I carried these stupid things around with me everywhere. It's a good thing there's no law against driving and reading flash cards!
I found the
MGMAT word translations book also helped with understanding what the questions were asking. Do you have the
MGMAT books? They're really priceless. I realize your test is in a few weeks, but if you still have time, I would invest in the books, or at least the subjects where you're struggling the most. There are some ridiculous rules that I probably learned in high school, but certainly don't remember now. Maybe that's our issue; we went so advanced in our math, that the little stuff they ask on the tests is long gone...
Hope that helps. Good luck on the test!