most of the time with ventress the reason she survived so long was because she was a coward so saying her surviving fights cant really be used
but i do think she is a better duellist but bastila is extremely skilled in the force ( i think the youngest jedi to of mastered battle meditation (something was not seen in any of the films.))
It's the fact that she has defeated Jedi Masters in combat before that makes her able to beat Bastila. Her fights with Plo Koon and Luminara were fights that she couldn't win, but she was able to do some damage. I doubt Bastila would have the same luck.
And yes, Bastila is powerful. But Ventress is in a whole different league.