There are a number of things that can be suggested but they all require you to put in some work to get real results.
Start writing. Write more and on wider topics. Do it without using digital crutches and do it on paper with a pen. Pick a random Wikipedia article in your target language (https://wikipedia.org/wiki/Special:Random) every few days and write a summary on it.
Take a picture of it or scan it and send it to a tutor or a native speaker to be corrected. Then write out the correction by hand. It’s very important that you don’t use anything other than pen and paper and maybe a native language dictionary. Try and do 200-300 words 3 to 5 times per week. For longer writing using this technique you need to do some preparation in advance.
Every week try and simulate the speaking examinations for C1/C2. Find a native speaker to practice with on any social media platform, and schedule a meeting. Pick a topic and prepare for giving a 3 minute monologue on that topic (2 and a half minutes at minimum).
Your listener should not interrupt your monologue with corrections but should keep track of them. Then you can do a quick recap on your errors. After that there will be time for your partner to ask you questions about the topic. You will easily fill an hour.
Prep as much as you can for this monologue. And most importantly, record these sessions, and go back and take notes based on the recording. Don’t take notes during. It’s just not worth it. Make sure you go back and review. You will find that much more rewarding, if you can handle listening yourself.
Try it out, and we’ll be back with more tips next week!