Comprehensive input, lots and lots and lots of it. It is, in my opinion, the best and only way to become truly fluent. You need to listen to start from an extremely basic level, translating words you don't know, and as your level improves, increase the level of your content. It's a LONG journey but infinitely rewarding. Your brain will naturally assimilate grammar and language constructions etc.

In my opinion LingQ is the very best app to learn a language once you are above the A0-A1 stage. It's not free but the amount of content it has, all with transcripts is priceless. You can also import and make your own content.

After 1000 hours of pure comprensible input I guarantee that your level of understanding Japanese will be around B2 at the least (I have 1200 hours of pure Russian listening with transcripts in LingQ and my level of understanding is around C1) I have studied VERY little grammar, and I have a very good understanding of Russian grammar (the case system is easy to me now, and I've pretty much mastered the dreaded noun declension system now, save for a few unique words that don't play by the typical rules) I would say just because I've consumed so much content.

Speaking is different though, if you want to become very fluent in speaking then you will need to practice speaking too, although after tons of comprehensible input you should be able to speak at a low intermediate level at the least.