Two months is a very aggressive deadline to comprehend N4 to N3 material.
A bit more information can help guide you to finding the right resources for your situation:
- How long have you been learning Japanese?
- Do you feel you have a strong ability to recognize and read ひらがな and カタカナ?
- Roughly how many vocabulary words do you know?
- How familiar are you with conjugating verbs and adjectives?
- How much grammar do you know? Do you know N5 grammar? How about N4 grammar? Or if you don’t know by Nx level, what resources (text books, web sites) have you used for learning grammar, and how far have you gotten? How well do you feel you know the material?
If you had more time, I would suggest downloading the Japanese Core 2000 deck for Anki to learn vocabulary. However, learning via spaced repetition takes time. On top of that, the more words you try to learn at once, the less well you’ll be able to retain them. The amount one can learn varies from person to person, but there will be a point where you take on so many in a day that you’re not able to retain them very well.
If you find following Genki, which is designed for a classroom setting, doesn’t get you the amount of grammar you need (or simply that working with a paper book doesn’t work well for you), you can check out web sites like Tae Kim’s Guide to Learning Japanese or the (from what I hear) more in-depth and complex IMABI for grammar. However, in order to understand the grammar you learn, you’ll need to become familiar with it, through repeated exposure. The sample sentences these resources provide will only take you so far. It’s a good start for someone leisurely taking their time, but if you’re starting from scratch with a narrow deadline, you’ll have your work cut out for you!
I recommend continuing to use WaniKani for kanji if you find it works for you, but it’s a fairly slow system because each level’s material needs to be remembered to a certain degree before you can learn new material, and the spaced-repetition system imposes a limit on how quickly one can do this.
For reading material, you can find a list of book clubs here at WaniKani. Typically, participants read a portion of a book (such as a manga or light novel) each week and discuss it. All prior discussions are available, so if you pick one of the manga to read through on your own, you’ll have accompanying vocabulary lists and discussions to help you through reading it. You may even be able to ask a question on an older discussion thread (if it hasn’t closed due to age) and get an answer by someone who read the book and is still subscribed to the thread. The book clubs include the Beginner Book Club recommended for people with N4 level vocabulary, kanji, and grammar, and the Absolute Beginner Book Club suggested for people at N5 level.
Good luck on your journey these next two months. I know I wouldn’t make it through all of N5 and N4 and into N3 in that time. (I started N4 grammar a month or two ago, and hope beyond all hope to complete starting to learn all N4 grammar by the end of the year…) Once the exam has come and gone, take everything you discovered about how you learn, and be sure to apply it for your continued education!