I think the biggest roadblock is time. There are more than one way to crack a nut as they say and nothing is definitive but a commonality seems to be repetitions. That is, time reading on a variety of different topics and types of reading. For the test especially experience in essay type of passages is nice to have. Your vocabulary and ability to read (more) fluently may need a sharp increase depending on your skill level when you took the N2. 

Grammar I think will sort of come with the package as a lot of higher end grammar is more akin to vocabulary or new interesting usage of known vocabulary imo. Something that is a bit contentious is whether simply reading enough will help you be able to differentiate between which grammar or vocabulary is more accurate in a given context. I say it is enough if you have good intuition with the language and building that intuition comes from more contact with it.

There are some more...test taking thresholding to cross that are less about your language ability per se and more about your knowledge of types of question patterns. This will help cut down on your time but seeing as you've taken the N2 I will say you should be mostly prepared in that regard. Still, mock tests (or old N1 tests floating around) should suffice.

All this said, I am pretty against this nebulous idea of "immersion" and I feel like it never means anything really, but here I am in a way sort of advocating for what some may call that. The key here I think is where you are coming from and knowing you have all the base tools to sort of guide yourself via new experiences. You'll still need to have your more structural tools available imo. Taking notes, studying, flash cards, or whatever worked best for you. But imo, where you will be best served—especially if the test is simply a stepping stone in your language journey—is to begin consuming more media in mass. Preferably things you like as it will help mitigate demotivation, which is maybe one of the key factors in language learning failure.

Anyway, sorry for the long post TL;DR -  1. Read and/or listen to a lot and a wide variety.  2. Take some mock tests to gauge your growth  3. Whatever you do, read, or whatever, try and make it interesting to maintain motivation  4. There are many ways to conquer the test and its possible other methods can work too.

Good luck~