Master Intermediate Grammar
Having mastered all of the N5 and N4 grammar back in Phase 2, you'll find that you're able to express a solid percentage of your thoughts in Japanese sentences now (or, if you've been slacking on conversation practice, at least be able to understand other people expressing thoughts in complex sentences).
That sounds like a ton of grammar, yeah?
Don't panic! I firmly believe that the most difficult grammar to understand is JLPT N4 grammar. So once you get to this portion of the course, you'll have gotten the most painful part out of the way.
The challenge with N3 and N2 grammar, on the other hand, is that there is just so much of it to learn.