I think you should focus on SC and CR. You should really understand all the rules in the SC part, you can solve most of the SC problems just by applying the rules. One part that is tough for non-native speakers is the idiom part and I don't really think you can improve in a short amount of time to really handle these types of questions well.
You can also improve a lot on CR, I'd suggest to get either the Manhattan CR guide or the CR Bible. It takes some time to get used to the GMAT logic though.
RC is the toughest part to learn in a short amount of time, you can try to read as much as possible, but if you lack the vocabulary it's tough to get through a challenging text like those on the GMAT. If you struggle due to other reasons, I'd suggest to read articles on science, economics, business, politics and history.