This is a shortish (remade slightly better) video to showcase a bug with the AI of the Ruin Serpent. It appears that if you get rid of the black mud puddle it was going to go to (after getting rid of one already) moments prior to it surfacing, it'll instead surface back at the close two black puddles, and just dive and resurface over and over until it's dead.
Uploading so I can submit the video in a bug report since they don't accept video, only images, in their reports.
Yes, I am well aware that I don't do beeg damage and I'm messing around to extend the time to show that it's just going to loop endlessly regardless of what I do- except eliminate the last two black muds which will allow it to continue.