Thanks, all - I have indeed now defeated Mother Nature. I went off to Mithalas for a bit, accidentally blundered into some sort of boss there, got killed pretty much instantly, and then made my way back to the Kelp Forest, found the Fish Cave, and finally back to Mother Nature, where after several more tries, and judiciously combining Toughness Cakes and Spicy Rolls (I think that was it? The one that makes your energy shot better), I finally did it! :-D
Now, to figure out if there's anything else useful I can do before going back to confront Mithalas-Instadeath again. But I am starting to feel a bit more confident about all this.