The final battle is surprisingly straightforward. There are three waves of enemies and three direct interactions with the Rimorosa. The first wave of enemies is jellies and urchins. Once you dispatch them, the Rimorosa comes to the foreground. Use your claw to latch onto one of the sections of its faceplate and pull it off. You should be able to do this twice before it retreats.
The next wave of enemies adds in anglerfish. Dispatch them and then repeat the process to pull off two more chunks of the Rimorosa's faceplate.
The final wave is Formori Sentinels. Unload all of your torpedoes since you won't be needing them any more and take them out, then pull the final pieces of the faceplate off.
Sit back and watch the final cutscene.