When you first arrive at Rusu Mountain. Talk to Beni and Saiya. Rusu Mountain start from Gorge Warp Stone (fast travel point). I have marked it on the map for reference. They will point to a wooden lookout high on top of the cliffs. This is your destination (Rusu’s location), where you need to go.
From the current location, if you swim forward and follow the left cliff towards the waterfalls. You will reach a rock that you can climb up and jump across the gap to find the Rot under a rock.
Return to Beni and Saiya and turn back. You will see a wooden archway on the right. Now it’s time to follow this path until you reach an area with glowing lanterns.
You will find a corrupted area. Defeat all the enemies and clear the corruption. Collect the Rot underneath the rock on the right. You have to use your Rot to move the stone.
Continue along the main path upward until you run into the spirit of Taro. Follow Taro’s spirit all the way to a Deer shrine, where you will be promoted to use a mask. After the quick cutscene that plays here, use the Forest Tear to move a Rot cloud through the corruption on the left. At the end of this pathway, you will find a Flower Shrine.
Return to Deer statue and follow the main path to the right. jump across all gaps to reach another wooden archway as shown in the below image. If you go left along the ledges on the left before the wooden archway, you will find another Rot.
Continue to follow the main path until you meet Beni and Saiya. They are looking over the cliff. There is a Ruins Warp Stone (fast travel point) behind Beni and Saiya. If you drop down using ledges where Beni and Saiya are sitting, you will find another Rot.
Now, comes the trickiest part to move forward. If you follow the main path as shown on the map then it’s not possible to move forward. Gaps between ledges are too wide to cross. To continue ahead follow the white edge paint on the cliffs – these white edge paint lines are your way forward through this section of the game. I have attached a series of images of how to move forward along the ledges.
While clambering through the cliff, you will find a cliff (last cliff) where you can follow the path to the left or go around the cliff to the right. Go around to the right and into a small cave to find the Spirit Mail.
After collecting the Spirit Mail, clamber through the cliff to the right to finally reach the main path. After climbing up all the ledges and clambering through the cliff, you will reach the main path again. At this point, go left instead of the main path. Use Rot to carry stone over to a high platform where a Rot is under a stone pillar.
Head back down to the main path and continue forward until you find a broken wooden bridge. Go to the right before the broken wooden bridge and jump to a higher ledge. Follow the ledge around the corner to find the Cursed Chest. Also, complete the Cursed Chest challenge to receive a Rot hat.
Now, use your Rot to pull the rope back from the broken wooden bridge, allowing you to successfully cross it. Proceed to the heavily corrupted Rusu’s House. Now, follow the glowing lanterns to the left to find the source of the corruption.
Destroy all of the enemies and the three corrupted bulbs. After you’ve destroyed all three bulbs, you’ll notice an entrance blocking your path forward.
Use Rots to move a small blue stone on the ground, and place it in the middle of the three broken pieces of stone in front of the door. Now To open the entrance, stand on the central circle platform and use pulse. I’ve attached an image for your convenience. Go inside the cave.
Once inside the cave, continue going straight and use your pulse to light up the path. Finally, you arrive at an arena where you must battle Kappa (Boss).
Kappa: Boss Fight
Kappa can only teleport between three reddish-black pools and fire red projectiles. Eliminate the minion and dash over to Kappa to attack. Once Kappa’s health is reduced by at least 50%, it will teleport to a higher location in the arena and launch a barrage of projectiles. Once Kappa is back to the lower level. Quickly take down all minions respawning and return to attacking Kappa. Repeat this procedure until Kappa is defeated.
After you’ve defeated Kappa, use your Rot to clean up the mess. This clears the way for progress. Continue on this new path until you come to an elevator. To begin moving up, apply a Pulse to it. When you reach the open area, follow Beni and Saiya to Rusu.