How do you deal with an executive that keep pusshing her work to you?

I report directly to the director. Only me and an executive in the office reports directly to the director.My director is a very busy woman and i seldom see her nor do i have a chance to communicate alot with her. She trusts me to do my job on my own. i do feel intimidated by her. However, the executive i work with makes things difficult for me. she will ask me to do menial task which by right is her work. I'm suppose to ist other managers in the office with their admin work apart from isting the director. But this executive keep pushing her work to me, then that work becomes 'my baby'. She claimed that she's very busy with isting the director and handling other projects. Both of us are supposed to cover each other duties when 1 is not around but seems like i do her job while she wont do mine. i feel at a lost because i cant bring it up to my boss, and this executive is known for her scheming ways. neither do i want her to take advantage of me. any advise?