Best to go for this post game if you missed it during your playthrough.
Go to Chapter Select and pick Chapter 2 Deluded Depths. Then select HMS Gryphon.
Stay at the top of the screen and spam the X button. You don't need to worry about the triangle button, I didn't use it at all. Watch out for the big fish who repeatedly comes from the left side. This is very easy, I got it on my first try.