To be in his element

Ser como pez en el agua. To feel very comfortable and be familiar with something.

Example:

Gloria is in her element when it comes to selling something - Gloria es como un pez en el agua cuando se trata de vender algo.

Come over

Venir (a visitar) - To visit , to come over.

Example:

Are you going to be free in the evening? Why don't you come over? - ¿Estarás libre en la tarde?, ¿por qué no vienes?.

My friend is comming over tonight - Mi amigo viene (a visitarme) esta noche.

By the end of the week

It means no later than the end of the week.

Cuando veas la preposición by y después una palabra relacionada al tiempo. Significa que no te puedes tardar mas de esa fecha,día, hora limite para hacer algo.

When you see the preposition by plus a time expression or word, it means that there's a limit to do something, you can do it on that date, day, hour or before, but you can't do it after.

Examples:

You have to finish the report by 4 pm - Tienes que terminar el reporte a más tardar a las 4 pm.

I'll come back home by Friday - Regresaré a casa a más tardar el viernes. (It means that I'll come back home either Friday or before, but not later).