Finish Learning Grammar
If you know the grammar for JLPT N5, N4, and N3, you know most of the sentence formations people use in day-to-day life.
Add JLPT N2, and you know most of the sentence formations people use, period.
JLPT N1 grammar tends to be a bit more obscure (although some of it is used in everyday speech). That said, you need to know it if you want to pass the N1 test, which is required for translation jobs (and just looks sweet on a resume).
We cover that grammar in our JLPT N1 grammar course: