How to raise my score from 700 to 750+ in 20 days?

You're already on the right path - picking up general techniques, as opposed to going through specific questions, is probably the best way to get to the upper end of the score scale. It looks like all you have to do now is make sure that you can make it through in enough time to do your best.

Practice can help out here; finish enough practice questions that you're familiar with most of the questions, and you will be able to get through them quicker. Skim RC passages to get the gist of what's going on and then head straight to the questions, since you'll need to reexamine the passages anyway. Memorise the order of the answers in data sufficiency questions, since they're always the same and that way you won't have to spend those extra few seconds checking the responses for each question.

Last, but certainly not least, relax! Don't worry too much about trying to fit into an exact time schedule. Worrying too much will just take up some of that precious mental bandwidth. Remember that the GMAT can be retaken as many times as you like, and that only the highest score counts!