a past master

past master

One who is exceptionally skilled or experienced in a particular activity, craft, or field. Typically followed by "in/of/at (something)." Not to toot my own horn, but I consider myself a bit of a past master in cooking. I don't blame you for feeling that way—Jen is a real past master at eroding your self-confidence. He's always been a past master of getting by with doing as little work as possible.
See also: master, past

a ˌpast ˈmaster (in/of/at something)

a person who is very good at doing something: He’s a past master at making other people feel guilty.
See also: master, past