He is seeing his best days―at the summit of his prosperity―at the zenith of his fortune―at the top of Fortune's wheel.
to pride oneself on―plume oneself on―peacock oneself on―pique oneself on―value oneself on―anything
He has a triumphant air
Pride is written in every line of his countenance
His face is beaming with pride.
He entered with a triumphant air―with a triumphant look―with a triumphant countenance.
He is strong in English―in his element in English―English is his forte―English is in his line.
He is in his element when he discourses on literature
Literature is his favourite topic.
Your present work is out of your line; you had better do something more in your line.
（客の方から言うと）to give one custom―patronize a shop―（店の方から言うと）―have some people for customers―enjoy the custom of some people―be patronized by some people.
That shop has foreigners for customers―enjoys the custom of foreigners―has custom among foreigners―is patronized by foreigners.
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strong [weak] in algebra - 研究社 新英和中辞典
one's best subject - Weblio Email例文集
Good at math - Weblio Email例文集