chief, senior
deputy
personal
I'll ask my personal assistant to deal with this.

special
administrative, care (esp. BrE), catering (BrE), checkout (BrE), clerical (esp. BrE), executive (esp. AmE), graduate (esp. AmE), laboratory, library, physician (AmE), production, research, sales (BrE), shop (BrE), teaching, technical
patient care assistants
She took up a post as research assistant in the Department of Pharmacology.

employ (sb as), have
She had an assistant to do her paperwork.

get, hire (sb as) (esp. AmE), recruit (sb as)
I'm getting a new assistant next month.

fire (esp. AmE), sack (esp. BrE)

assistant to
She is assistant to the Production Manager.

Assistant is used with these nouns:
attorney, bishop, clerk, coach, commissioner, counsel, director, editor, gardener, head, lecturer, librarian, manager, principal, producer, professor, referee, secretary, trainer, treasurer, tutor