Bullying is very subjective and many organizations have a policy about it. Better check it out. Other thing is, make sure that you don't signal your intention as he or she might get a case against you earlier.

Anyway, chances are it will be your word against her. You don't need a video because it is actually illegal to video someone without their permission. Just keep a diary and perhaps get a feeling from colleagues if everyoen feels the same. The more to complain the better