Study technique: Focusing on FRA, Equity and one other large topic

Hi everyone. I have heard that some people recomend having a “portfolio” of 3-4 topics that you know really well (usually FRA, equity, and for example corp finance) where you can score 80%+ on the exam and then knowing the main concepts of the rest of the subjects where it’s ok to guess/have 40-50% on the exam. Do you think this is a good technique? Has anyone tried this? Thanks