The creature's wiles were far from limited to tossing shit like an ape. It hadn't really expected the rock to hit. That would be too easy. Not to say the rock wasn't important in the overall plan. Afterall, the only way to hit someone that can vanish at will is to hit them before they have the will to vanish. To do that a distraction was needed, hence the rock.
At about roughly the same time Crual moved to avoid the rock, when his attention was on the blur of the stone, the Magna Pater's prehensile tail flicked upwards like an inverted scourge. The very tip, muscular and flexible, was curled about the middle threads of the net which it had been pulling along beneath the tail. The tail couldn't reach its target, but what it was subtly holding could. There is an old torture weapon known as a cat o' nine tails. A whip with multiple heads, and small stones or glass tied into each strip of leather. Such a thing would be only partially comparable to what now was sent lashing up from beneath Crual, for it was twice as long as any nine tails whip and the jagged chunks of basalt glued to each strand, still very sticky on the edges and underside which the monster had cleverly not slimed, were of vastly greater size and heft than the paltry bits of debris found in the impromptu weapon's namesake.
It could very well take a miracle for the elf not to get absolutely clobbered by the big mess of stone laden strings, uppercutted hard enough to put him out of his senses and redirect his fire up at the roof of the tunnel above his head, perhaps making him even blow himself up. Even if the elf didn't get exploded, it would take but a second or two for the monster to jerk his stuck and stunned personage in close for the kill. And even if that didn't occur, of all things, there was always the consolation prize which the beast had set its mind on attaining. The ranger's bow. It hoped to at least tear it from the elf's hands with the suddeness of the sticky, entangling strings before the one holding the black bow had a chance to phase away with it, soiling the aim and taking away the most important tool in its foe's arsenal. Certainly it would be an improbable feat to keep a straight aim when getting ragged by a giant flail.
It was a tricky beastie that relied as much on smarts as strength, that had been obvious from the beginning. But perhaps now it would be clear just why in over a thousand years, not even the utmost efforts of the wisest hunters and boldest heroes had bested it.