Hi jjwclub,
Since you have just 16 days, a great way to improve your skills is with topical practice. For example, let’s say that you want to practice Number Properties. You can do so by answering 50 or more questions just from Number Properties: LCM, GCF, units digit patterns, divisibility, remainders, etc.
After each problem set, thoroughly analyze your incorrect questions. For example, if you got a remainder question wrong, ask yourself why. Did you make a careless mistake? Did you not properly apply the remainder formula? Was there a concept you did not understand in the question?
By carefully analyzing your mistakes, you will be able to fix your weaknesses efficiently and, in turn, improve your GMAT quant skills. Number Properties is just one example; follow this process for all quant, verbal, and DI topics.
For some more advice, here is a great article you can check out:
How to Improve Your GMAT Score