Only heavy attacks with Y seem to affect the beast. You can also use focus and get hits while in your focus. The beast will run off after landing some hits, you will have to dodge out of his way as he runs back towards you. This will happen multiple times. Just keep hitting him with held Y attacks and using your focus when you can. Soon enough, you will kill the beast after fighting him in the dark. And thiis will unlock the Achievement.Complete walkthrough of this area:
Soon after you enter Helheim and unlock the “Into the Mountain” Achievement, you will see a Lorestone. Focus on it to work towards the “Stories from the North” Achievement.
Now begin your path inside Helheim by taking the left passage across a bridge and open a door. After some uncontrolled running gameplay, there will be a brief cutscene with
followed by 3 symbols on a doorway that you must find.
Turn around and take the first left down a spiral staircase. You will have to go into a water canal and you will end up losing your torch. You will then need to run to areas of light until you find another torch. Return back to the dark areas, lighting torches along the patch and you will find your first of three symbols for the door. You know your near the symbol because you will see floating red symbols like in previous symbol puzzles. The first symbol looks like an English letter P and is found beyond a wood archway. Light the torch to get a wood structure to reflect in the water, making the symbol.
After finding the first symbol, return to the next spiral staircase and walk up to open a door. There is also a Lorestone nearby. After opening the door, the main door with the symbols is to your immediate left.
Now for the next symbol, turn right from the door and follow the pathway all the way to the second right pathway. You will go down a stone stairway and you’ll encounter a handful of enemies (the only jumpscare I’ve had in the game, kinda scary). Kill all the enemies and there will be a blood splatter on the ground that makes the second of three symbols. The symbol resembles a fish. Light the torches and walk up a pathway to focus on the symbol. Walk out of this room and back to the main hallway to find symbol three.
Return to the spiral staircase that went down to find Symbol one, but this time go up the staircase and follow the path and you’ll eventually find a room with the glowing/floating symbols. Light the torch to create a shadow that puts the third symbol on the wall. It resembles an English P (again). After focusing on the third symbol, follow the path into the dark hallway. Open the door, turn right and then right again to find the main door with the three symbols. Go and open the main door. Now the beast is right behind you, press LB and begin running down the hallway until you get a cutscene.
After the cutscene, follow the path to the right by walking past the tight walkways. Enter the doorway and pickup the torch (press A near the torch). Follow the path until you must cross a waterfall along a wood plank. You will see a symbol that looks like an English letter R. Walk up to it and you will fight multiple enemies.
After killing the enemies, pickup your torch and take the pathway to the right of the symbol and light the cauldron. Go back to where you entered from, and you will begin to see the glowing/floating symbols. The wood near the cauldron you just lit will be the symbol. Focus on the symbol and return to the door and open it.
Follow the pathway along under a waterfall, losing your light. Press and hold LB to run until you find another torch. Light your torch and follow the path until you see a beam lit up. Walk across the beam and follow the pathway down to the left (to the right next to some metal cages is a Lorestone).
You will see another door lit up red with a symbol. This symbol resembles an English letter M. Light up the cauldron near the door. You will see a waterfall to the right of the door, but before walking through, return to where you dropped down a cliff to enter this room. To the left you will see a cave. Enter this cave and follow it until you can light a cauldron. Return to the main area, walk under the waterfall and press and hold LB to run to where you just lit the cauldron and you will be able to light up your torch after crashing into a bridge. Return back to the waterfall and light the cauldron next to the waterfall. Go back under the waterfall, light your torch and focus on the wall up find the symbol. Open the door.
Follow the path until you fall down a hole next to some bones. You will move past a bunch of arms reaching for you (creepy), and eventually crawl through a tight cave hallway. You will then need to run until you see a blue light and the beast, followed by a cutscene.
Now begins your fight. Only heavy attacks with Y seem to affect the beast. You can also use focus and get hits while in your focus. The beast will run off after landing some hits, you will have to dodge out of his way as he runs back towards you. This will happen multiple times. Just keep hitting him with held Y attacks and using your focus when you can. Soon enough, you will kill the beast after fighting him in the dark. And this will unlock the Achievement.