Defeating the pill-shaped thing with briar thorns proved to be remarkably easy. I didn't count the number of hits the thing took, but it took me no longer than five minutes to kill it. The trick is to position briar thorns at the concave sections of its rock, so that it takes hit from the same thorn for longer. Hitting the pill-shaped thing on the convex sections is pretty difficult, as the briars spread out and it only takes a couple of hits before moving on, as opposed to the 5-10 it takes from a concave section.
What it drops is either a Divine Soup or a Royal Soup, I couldn't tell which from the picture and had both in my inventory at the time, so it could be either. It's not a unique drop, however, as I'd gotten one before, though I can't tell you from where. It's rare, at any rate, and a recipe you get FAR later in the game normally.
Guess that's one mystery solved
