Have you been reviewing the flashcards from this site? No joke - I firmly believe that SC is the easiest Verbal Question type to improve, especially in short periods of time.

Is English your native language?

As you go through the flash cards, set aside each idiom card and tackle those more frequent. Saying them aloud will help you remember. It will get you a few more correct as they become reflexive. Also, keep doing the OG SC questions. The repetition is what drills each error type/correction to mind.