Master
Expert
noun
A person who has "attained" is someone who knows much. They are a master or expert, obviously.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
彼はカメラの達人です。
He is a master of the camera.
彼女はウチの会社の達人プログラマーです。
She is a programming guru for our company.
ウチの母親の料理は、料理の達人レベルに美味しいです。
My mother's cooking is delicious to the level of a culinary master.