Putting up with rudeness at work?

My 'istant' is also the istant of my boss. (note the unfair heirarchy from the start). The istant is very popular - people feel protective of her - she is viewed as an underdog. Unfortunately, she is very rude when I delegate work, and is spreading untrue rumours about me. As I am not an underdog (some would cl me as a tall poppy) my complaints about this woman are not being taken seriously, so I am actually the one being disadvantaged. What can I do? I need her help to get my job done but she spends more time on the big boss's work and then complains about me when I get upset if something isn't done. The boss is siding with her. I might add I am the only woman in the office who is not obese.