I haven't yet reached C1 in my target language (Spanish), but I have been listening to TV shows and writing exercises here on iTalki, which gracious people then correct for me. Because of that, I have begun to write more grammatically correct Spanish naturally, without specifically trying to follow any rules. I still make mistakes, but somehow I'm starting to get it right more often, even though I haven't been studying grammar from books.
Reaching that kind of "C1" mastery also means knowing a lot of words too, right? I've also been intentionally memorizing hundreds of new words using flash cards.