With a little Googling, I have worked out that the term is 고인물, which translates to "pooled water." This Korean gamer slang for someone who has spent an enormous amount of time perfecting their skills in a given game or discipline. Check this Reddit thread for a full explanation: https://www.reddit.com/r/Korean/comments/c2pnf2/korean_slang_of_the_day_%EA%B3%A0%EC%9D%B8%EB%AC%BC/
I think "master" is too vague a term, because it comes with all kinds of connotations. It implies that he is in a position of authority over the speaker, which is completely not true. On the other hand, "advanced player" just...doesn't really mean anything? Neither is a great option.
Personally, I think a better term would be "pro gamer" or "pro player." This in English-speaking gaming culture has taken on a very similar meaning as 고인물, and English-speakers will often call someone a "pro" or "pro gamer" not to literally mean they do it for a living, but as a reference to a high level of skill attained by grinding a game for thousands of hours. It's also a term that is used jokingly outside of gaming in other fields, like calling a skilled maneuver in a sport a "pro gamer move."