MGMAT quant is cumbersome, while GMAT quant is full of traps. So you should use the strategy guides to learn the concept and solve related problems in OG. Once you have solved the problems, be it right or wrong try to search for different ways to solve that single problem - post it in forums, search through forums, find expert solutions. Ideally you should take 20 mins to solve 10 problems and 40 to 60 mins to review the same 10 problems.

The next thing you should do is to check in which section you are making more mistakes, whether DS or PS. PS tests conceptual knowledge while DS tests your ability to classify the problem according to the information given rather than finding a proper solution. To avoid more mistakes in PS requires improvement in conceptual knowledge, while to avoid mistakes in DS, you need to revisit the strategy. Use ADBCE approach, solve more problems and review in the same manner mentioned above, you will notice lot of patterns and shortcuts. Make flash cards/notes, prepare error log to include lot of shortcuts such as in an evenly spaced set - the mean and median are equal, while facing inequalities you should be wary of signs (whether +/-ve nos are included or not). The less you solve and more you review, the more you gain shortcuts and tips to solve DS. DS can be improved only with practice. Don't give more importance to hard probability, combinatorics problems they are less frequently tested in GMAT. You should start extracting every ounce of tips/shortcuts, which can be applied to other problems, from every problem you review - this should be your ultimate goal. Solving tough problems won't help you increase your scores. Hope this helps !!