At C2 level you can understand with ease virtually everything heard or read. You can summarise information from different spoken and written sources, reconstructing arguments and accounts in a coherent presentation. You can express yourself spontaneously, very fluently and precisely, differentiating finer shades of meaning even in more complex situations.
You can advise on or discuss sensitive issues without awkwardness, understanding colloquial references and dealing diplomatically with disagreement and criticism. You can hold your own in formal discussion of complex issues, putting an articulate and persuasive argument, at no disadvantage to other participants. You can deal with hostile questioning confidently, hold on to the turn and diplomatically rebut counter-arguments.
You can negotiate complex or sensitive transactions in public, professional or academic life. You can keep up your side of the dialogue extremely well, structuring the discourse and interacting authoritatively with effortless fluency as interviewer or interviewee, at no disadvantage to other participants. You can use telecommunications confidently and effectively for both personal and professional purposes, even if there is some interference (noise) or the caller has a less familiar accent.