how to detect what part of the body was damaged?

I'm making an Attack on Titan datapack and in the series, titans have a weakspot on the back of their neck. if my datapack was made with only pvp in mind that would be okay since I could just use interaction entities in that area for if a player hits it. the only issue is that you are also able to turn into a titan, meaning mobs have to be able to somehow hit this weakpoint. thoughts?