Αυτός είναι κορυφή is not right. We use it, but is slang. The correct word is: κορυφαίος.
Greek doesn't always use the article in this case. I'd use "στην" but it's ok as it is. I also like "He is the best." or even "he is best."
Could this also mean "he's an expert"? I'm asking because in german we could say "Er ist eine Koryphäe" (meaning "he's an expert" and "Koryphäe" is derived from greek κορυφή).
It really means "the top" as in a mountain or as here "ability". So, by extension it would mean ''expert''. In Greek, you will also hear "εξπέρ". :-))
This is slang! Fantastic! Say that to a Greek guy "φιλε μου είσαι κορυφή" and he 'll buy you drinks all night long :-P . That's what I love on Duolingo
similar slang expression "είσαι κορυφαίος=you are the peak/the top/the elite/the best"