Thanks for your question and yes, you're in a difficult position from what you describe.
The best way to safeguard yourself while getting the problem to stop is to send an email or another written communication to the most senior exec you feel comfortable addressing; copy another senior exec and the most senior HR professional and say something along the lines of:
I believe I'm experiencing harassment based on the actions and comments of [name]. Here's what I'm experiencing: _____________.
I realize this person is a senior executive and has the support and goodwill of leadership. Even so, can I get written assurance from one or more of you that I will be protected from a negative job action that may be triggered as a result of my expressing these concerns about [name]? As I'm sure you can appreciate, it's not easy to express these concerns about such a powerful person within our organization and I'm only doing so to remedy a bad situation. It would be great to get the written assurances to raise concerns about anyone in our organization, regardless of how senior or powerful.
Workplace & Harassment Expert
February 9 2018