I can share with you some of my English learning experience.
From B2/C1, if you want to improve, you can try the following suggestion.
Learning phrasal verbs
Learning some vocabulary in different area(Medical terms, cooking, history, or even legal English)
Practice speaking with Native, and get used to the way they speak, understand their slangs.
Try to speak English to English learners from all around the world, understand their accent.
You can learn like a child --- imitate! just pick some good audios/videos, and try to repeat what they say. Good idea?
That's in addition to speaking with native speakers of course.