I had heard the middle cave was the worst...but I found it to be the easiest. Took me less than two minutes.
In my typically cowardly way, I hovered near the entrance to the middle cave with a load of shark and my shovel, ready to dash back out at the first threat to my nice shoes. When the beast quickly released the first core, I was pleasantly surprised to find that the beast stopped attacking. I was also surprised to see that I was being attacked now by the core, even tho it was kind of like being buzzed by a mosquito. When I dashed back out of the entrance to calm down, the core just waited for me. I ate my shark, ran back in and dug at the hole nearest the entrance, and the core jumped in. It was great. Just let the core attack you, it's not bad at all. Run back and forth near the entrance (like you did in the first room) till the core is released, duck out the entrance, eat, run back in and dig a hole at your leisure, because the beast will not attack you while the core is on the ground. Voila, you are done. I had enough shark left to head straight to the third cave.
First and third caves, you are on your own. ~:> Timing is everything. Salmoneus.net has a great Summer's End guide for the first room. Third room is different because the beast's attacks are different, and not so bad. I just died once in the third room. Again, just duck back into the second room to rest and refresh. The beast will be waiting for you at the same point you left. And if you die, just use your games necklace to get back quickly to the portal at the farm where all of your sharks and stuff are waiting for you, and the ring in the rubble next to it. Very cool.
I bet a lot of you already knew this, but if you didn't, hope this helps. Scary quest. But if I can do it, so can you. ;) And thanks, Beef, for the encouragement during the quest!
, Guides all said to build all the pyres in the first room, before lighting. That just didn't work for me so I did it one at a time with success and only one death. The last two rooms are much easier 
