When you reach the conveyor belt next to the water boss, run along to the left and kill all the humans. This will trigger the cutscene with the tentacles attacking.
Keep moving left and attacking the tentacles, dodging past when one lifts. At the end the camera perspective will change and you will be on a tentacle.
Press A and up on the left stick to climb quicker, pressing X to kill the reachers.
If you get caught by the closing claws, bash B to escape.
When you reach the top of the tentacle follow the button prompts.
After the cutscene, fight the enemies and jump across until the beast attacks again. Go to the four corners of the platform and attack the tentacles. When all four are released head to the edge of the platform nearest the beast and follow the button prompts again to defeat it once and for all.