Should I rewatch a specific piece of content over and over to master it?

Fluency comes from building language proficiency through lots of interesting listening and reading. If the content is so wonderful that you want to go through it again, that's fine. But as far as language acquisition goes, I think rewatching or rereading is a waste of time.

Rewatching the same episode over and over isn't the best approach for two reasons. First, at any given level, there's only so much unfamiliar language you can pick up. We call this the "low-hanging fruit." Some words and expressions will remain out of reach and can't be acquired no matter how many times you rewatch. As your understanding improves, new low-hanging fruit becomes available, but repetition alone won't bridge that gap. Second, to acquire a word or phrase, you need to encounter it many times in different situations. That variety helps you get a fuller picture of how the language works. Rewatching one episode limits that variety. Moving on to something new gives you more opportunities to see words and expressions in new contexts and fully acquire them.

Another thing is that if you force yourself to rewatch the same piece of content, it will probably stop being fun. And boredom gets in the way of language acquisition.

Instead of rewatching a specific piece of content, like an anime episode or movie, we recommend watching it once and then listening to it during passive immersion. Usually, you'll have multiple tracks like this in a playlist, and each one gets played once in a while in random order over a period of ten days. After that, it goes out of rotation. Ten days is enough for you to get the most out of relistening to the same piece. Then you replace it with the next thing you've watched.