I’m not sure I can provide more useful or specific advice than @FlamySerpent, especially since I haven’t taken any JLPT paper yet, but if you want some thoughts based on my journey from a decent B2 (~73%) in French to a near-pass for the DALF C2 diploma in French (47%, mostly due to a misunderstanding about what might count as plagiarism – I think I would have scored at least 60% if I had done it the right way) within a year, followed by a good score for the DALF C2 (84%) half a year later (that makes a total duration of 1.5 years), I guess I can offer some advice:
If you’re sure that your course covered most of the N2 based on lists of N2 grammar and vocabulary you’ve found (doing all that in one year with students entering the course as beginners is really rare for any organised course, by the way), then good for you. If you’re comfortable with all the material that was covered, then I think you should be at a point where you’re able to read even news articles quite easily, vocabulary and perhaps specific kanji aside. In that case, you should probably do tons of immersion and read lots of news articles or listen to TV programmes meant for natives, because you’re not going to go very far beyond N2 grammar points without such immersion unless you buy JLPT grammar books.
However, I see that your self-evaluation leads you to conclude that
In that case, immersion can still be useful: very sincerely speaking, I learnt almost all N2 grammar by watching anime, looking for transcripts, and looking up all the words I didn’t know, especially grammar/common phrases. My level at the time was also roughly half of N3. However, to progress faster, you’ll probably want to try referring to an intermediate textbook like Tobira, An Integrated Approach to Intermediate Japanese, or any other intermediate textbook that suits you. (Samples of these books are usually available online.) That aside, you might want to get hold of a JLPT-specific book like the N2 book of the Shin Kanzen Master series, so you can practise for the tricky things that might come out on the test, and get an in-depth explanation of specific grammar points.
Just a few other thoughts:
Immersion and textbook learning offer greater speed, but in different ways: textbooks speed up your study of basic essential knowledge by compiling all of it for a particular level and presenting it to you in a format that’s usually easy to consume. However, they probably won’t habituate you to native usage, except at the higher levels, and won’t prepare you for fast speech or casual pronunciation, which is often more difficult to decipher. They also won’t prepare you for slang. On the other hand, immersion will bombard you with tons of common usage and new words specific to the particular type of material you’re being exposed to, and will help you get used to real usage more quickly, along with providing you with new words more quickly than a textbook, while also making you aware of what’s actually common in real usage. However, the knowledge you gain won’t be structured, and you’ll need to organise it yourself. In short, textbooks are a good way of getting you off the ground fast, but to keep flying and climbing, you need immersion and practice to help cement your knowledge and to broaden it so you’ll be able to communicate in various contexts, and not just in relation to what appears in textbooks.
If you really want to learn fast – even though your knowledge might end up being a little skewed towards complex/literary structures – then aim to transition to a monolingual dictionary (yes, Japanese only) as quickly as possible. You don’t have to do it in a single jump; it’s OK to rely on an English-Japanese, English-Portuguese, English-French etc. dictionary when you get stuck on a monolingual dictionary definition that you can’t understand. No problem. However, do your very best to rely more and more on the monolingual dictionary over time. There are two major advantages to this:
- Monolingual dictionaries contain more detailed explanations. They’re very precise. Bilingual dictionaries often do little to explain nuance, and tend to just throw translations around even when they’re not all that exact (sometimes because translation is nearly impossible). They also often include common phrases – like a verb that a particular noun always appears with – in the examples section that you won’t usually see in free bilingual dictionaries.
- If you read a monolingual dictionary, then even your look-ups become reading practice. Also, you get exposed to tons of structures that are used for explanation, as well as synonyms in Japanese for words you already know or need to know. This augments the immersion advantage: you pick up way more words per study session than someone who uses a bilingual dictionary. Proof that this works: I was the only person in my entire advanced French class to use a monolingual dictionary all the time, and despite the fact that just two years prior, many of the people in my class had better grades than me, whereas I had just 56%, I eventually graduated top of the class (89%+) with a C2 diploma in my pocket, and that was after skipping half a year of French class to improve my chemistry grades and not studying for the final exam.
To sum up, my recommendation is to work on core vocabulary and grammar as quickly as possible, then to start adding as much immersion time as you can, possibly while moving on to an even more advanced textbook so you can start heading towards a high N2 or even the N1. The point of studying core knowledge is to get yourself to the point where immersion is manageable and doesn’t require you to struggle through sentences because of grammar or a severe lack of common vocabulary. In the initial stages, immersion is a very slow way to learn. Afterwards, however, immersion becomes the fast track, provided you focus on a few broad areas at a time so that certain words will have a chance of coming up often enough for you to master them with little effort (e.g. I love scientific research, especially biological research, so I constantly read French news articles about them, which helped me with scientific vocabulary).
Finally, here are some resources you can use for this:
NEWS WEB EASY Simple articles from NHK
NHKニュースサイト 日本全国・世界の速報、最新情報｜NHK NEWS WEB NHK Articles for the general public in Japan
NHK for School if you want to try immersion based on what Japanese schoolchildren might study (it goes up to high school level)
https://ejje.weblio.jp Weblio’s EN-JP dictionary, which is better than Jisho in my opinion. It contains fully translated example sentences, and you can even search for phrases
https://dictionary.goo.ne.jp Goo’s monolingual dictionary, which includes a thesaurus and a simple EN-JP dictionary in other tabs, as well as a bank of example sentences for each word
https://weblio.jp Weblio’s monolingual dictionary, which includes dictionaries that cover slang and dialect usage
https://kotobank.jp Kotobank’s monolingual dictionary, which includes one of the same dictionaries that Goo and Weblio provides (大辞泉), along with one or two dictionaries that I think don’t turn up on Weblio
If you enjoy anime, then you should also try looking for transcriptions on Anicobin by searching
‘[anime title in Japanese] [episode number in numerals e.g. 1, 15, 21]話 anicobin’
You should get a reaction page containing screenshots captioned with dialogue from the anime, and Twitter comments from Japanese viewers. It’s good for helping with listening practice using anime, looking up new words, and for learning Japanese slang by reading what viewers say.
With that, I wish you all the best. I hope you manage to learn everything in time!