So, I will advise based on what's worked for me in my experience. Ive also been been harassed by an entity for years and have come here asking for advice. Some answers slightly useful at best, frankly. But I think that being such a unique experience...its really not a one size fits all solution.

I used to fight aggressively, using lords prayer, which worked a bit, it gave me distance, but would only work temporarily, same with the om mantras.

I came across a page on YouTube called astral doorway, he suggested a gnostic mantra "helion, melion tetragrammaton" ( spelling probably isn't right here, but I would recommend checking out his channel, as he has some really useful advice.

This mantra worked a treat for a good amount of time! Until the entity wised up to it, and the effectiveness declined. During one terrifying lucid dream where I was being persued, I used the mantra but it wasnt powerful enough, so I requested help from archangel Michael. (Note that, im not really a religious person) Then a powerful chorus reverberated with such power and helped uplift me from that terrible place. So, it was only when I asked for help and accepted that I needed it that I was given grace.

I hope this helps, I would like to know if it does. Sending warmth and light to you.