Master
Expert
noun
A person who has "attained" is someone who knows much. He/She is a master or expert, obviously.
たつじん
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.
一つのことに全てをかけないと、真の達人にはなれないぞ。
If you won't put all your eggs in one basket, you won't be a real master.
ビエトは、カメラの達人です。
Viet is a master of the camera.
父は、ピザ配達の分野で、世界レベルの達人であると評判になっています。
My dad is famous for being a world-level master in the realm of pizza delivery.