Possible spoilers so, beware!
So, while I was exploring I came across the Lerion estate and its secrets.
I found out that, while you are fighting Regan, you can bug her out of the combat zone, making her not attack while you can hit her with arrows and abilities.
I was fairly low to fight her.
Here is how it happened:
I placed myself facing the path between the statue (how you start the encounter) and the cave where she "might" spend the nights.Managed to get her between me and the wall on the left to the pathUsed the "Rush & Bash" ability on level 2 to pin her to the wall and somehow she just stayed completely stillTried to use the bow and she got damage (also with the Axe throwing ability)She did not went on second stage of fightKilled her
Things to consider:
If you move out of the combat zone and she is fairly away from said zone, a small shield on the health bar appears and you cannot hit herShe is pushed back everytime you it her so it is possible that you are unable to give damage to her if she is further away and you need to move out of the zone
Unfortunately I did not manage to record the encounter so it would be easier to understand.
For gameplay sake, Ubisoft, patch it so she can teleport back inside the zone when it occours.
Before they do, Vikings, try it.