At very advanced or Cambridge Proficiency level (C2) you will certainly be able to express a wide range of ideas pretty effectively but you will still be making some errors with more difficult constructions and new vocabulary. As your English gets to proficiency level, expert users of English will stop filtering their English so fully understanding what they are saying and writing will still be an issue.

Try some advanced level English lessons

Tips:

• Try  to expand your awareness of multiple meanings by checking new words and phrases in context from this website.
• Aim to extend your knowledge of vocabulary while reducing your errors in grammar and pronunciation.
• Increase your sensitivity to the appropriateness of your English. Feeling the difference between slang, ‘neutral’ and formal registers takes time.
• Decide how much you want to acquire a ‘standard English’ pronunciation. At this level some learners want to keep their cultural/national identity when speaking English.
• Keep working on stressing the right part of words and sentences.
• Read novels (native reader level) that have plenty of dialogue. Detective novels can be very suitable and motivating.
• Keep up with news and current affairs so you add to your vocabulary and have something new to say in conversations. Traditional newspapers can be helpful but be aware that they tend to be full of cultural references and vocabulary used in special ways..
• Use exercises from Cambridge English: Proficiency to test what you know. Browse the Internet for free exercises.
• Try your best to widen your circle of expert/native users. They will help motivate you and expose you to new aspects of English.
• Be disciplined to review your ever-growing vocabulary.
• Pay particular attention to collocations and expressions/groups of words.

Try some advanced level English lessons