The words that carry you from surviving to actually following along: society and news, business basics, feelings, school life, and the connectors that show change and degree.
From N3 up, the exam is scored out of 180 points across three sections instead of two: Language Knowledge/Grammar (0-60), Reading (0-60), and Listening (0-60). N3's overall pass mark is 95 out of 180, but you also need at least 19 points in each section individually, so a strong section can't cover for a weak one.
That means every minute you spend memorizing grammar has to actually earn points on test day. Some particles are simple enough that a little study locks in the point almost every time. Others have so many overlapping jobs and native-speaker gray areas that even hours of study won't make you meaningfully faster than a guess. Spend your limited prep time where it pays off, and treat the rest as a quick guess you move past.
If a question hinges on one of these four, take your best guess and move on. Chasing certainty here costs more time than it's worth.
Around N3 is where flashcards start hitting diminishing returns. You already know enough words to have real, if clumsy, conversations, so from here the fastest way to keep growing your vocabulary is to actually use it: talk with a language app, an AI chat partner, or a real person, and let new words come up naturally in context instead of only drilling lists.