a 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.