I have a colleague who picks on me on and off over the past year whenever I am alone with her. I tried my best to avoid her it's not always possible. She would claim that I am doing something wrong and threatened to report me. Whenever I checked with my supervisor if I was doing anything wrong, they said that I wasn't and not to worry. Apparently, she never follows up on her threats to report me and seem to only be on a power trip. 

I tried politely disagreeing, ignoring her, calling her out, speaking to lthe union and speaking to management, but they were dismissive and said not to worry. I think the reason that nobody believes me is because she's exceptionally friendly to everyone, especially me, when in public, but it's all fake because she's constantly talking behind people's backs as well. She also bootlicks managers frequently to get in their good books. 

The last thing that I am to do is to report her to the agency we work for, who will likely sack her immediately. They are aware of it but I opted to wait and see if she does it again, as it is frowned upon to bypass management and go straight to the agency. I would have to go through the entire grievance process again if it reoccurs. 

I have been assigned to sit with her despite making it clear to management that I feel uncomfortable. If she picks on me again, WIBTA if I tell her "Stop being a bully and apologise to me right now or else I'll report you to the agency. I hope that you get sacked." and write everything she said down in front of her? The last line sounds asshole-ish, but I'm aiming to intimidate her into stopping, but would that make me no different from her because that's exactly what she said to me? I'm afraid that she will twist my words and make it seems like I'm bullying her instead. 