Lol Sam, you are full of compliments today! Likewise I can say, "When it comes to asking questions about the English language, Sam is in his element." "When it comes to giving no nonsense answers to Italki questions, Eliot is in his element."

A fish in water is a fish in its element. After all, the fish's element is water. Our element is air. You function quite easily and naturally when you are in your element. I seriously can't say if teaching is Jura's element. I would have to take lessons from her and get to know her character a lot closer. You really need to know someone very well before you can state the he is in his element or not. Try not to overuse it. From thefreedictionary.com in one's element Fig. in a natural or comfortable situation or environment. Sally is in her element when she's working with algebra or calculus. Bob loves to work with color and texture. When he's painting, he's in his element.

It's when you're doing something you like very much and are comfortable with. The opposite meaning is expressed by out of one's element

Doing what one is good at.