what is his weakness?

So, maybe I suck in this game, but combat is too hard and unforgiving. Every enemy one shot you and there isn't much checkpoints, so you replay a lot. Your knife is small, so it is almost impossible to hit anything and even if you hit it, there is very limited stamina. Eel boss is just frustration and I can't get to it and kill it. Somebody have some useful tips?...

It just clips through walls, keeps yelling, the black light doesn't do anything to the purple on the diver and I get stuck on the first piece because there is no area to hide????? Story was cool until now, it just gets so frustrating I couldn't care less for the story........

How do I "End its suffering" with the eel? Do I need to hit all the purple spots on its head in one shot? That seems impossible.

Game doesn't progress after you shine your light on the intercom.