N1文法 is a video series from the 日本語の森 team that's designed specifically to help Japanese learners prepare for the JLPT N1 test.
日本語の森 is a fantastic YouTube channel designed for learners of Japanese at any level. Each of their videos is carefully planned and presented by Japanese teachers. While they've had (and still have) series aimed at the JLPT 1 level before, this series seems more focused, with fewer jokes, though perhaps less charm also.
These videos will help you prepare for the grammar you'll face on the JLPT N1 test as well as test your listening and general comprehension skills.
The videos range from around thirty minutes to an hour and thirty minutes, depending on the grammar points that are being covered. Each grammar point is presented on the screen behind the teacher, while they provide explanation and read example sentences. The entire video is in Japanese (as most of their grammar videos are), using word choice and slow speech to help make everything clear.
All of the text shown on screen is also available in the description for each video—including every single example sentence! I highly recommend inputting the sentences into your SRS tool of choice after watching each video and drilling them until you know the grammar points and their rules by heart.
These videos will help you prepare for the grammar you'll face on the JLPT N1 test as well as test your listening and general comprehension skills. And while the audio quality still seems to vary, the content they're putting out is absolutely worthwhile for everyone preparing to pass that final, hardest level.