The Turbo is relatively safe if it's sealed up in its shell, although some predators have different strategies for killing them anyway. Some can drill the shell using acidic saliva in combination with their radula, and some cover the aperture and use the "pry" method to slowly wedge the shell open as the prey animal tries to extend to move or breathe, or as it tires of pulling against the attacker. Most attacks are chemical, though, in combination with the tactics I just mentioned. Many species, not just the Fasciolariidae, produce chemicals that paralyze their prey, or anaesthetize them to the point that they can't resist predation. Some then inject digestive enzymes that dissolve the prey animal so that it can be sucked out, while others simply extract the animal and swallow it. I'm not sure which tactic your snail uses, but it probably attacks the snail chemically as it opens to breathe, having covered the aperture, and forces it to breathe in the paralytic toxin. I'd guess your snail then extracts and eats the animal without further chemical assistance, but I might be wrong. I'll see if I can find out which is most likely.
Of course, the predator snails can't be overly successful at killing a species, or they'd wipe it out. Conversely, the predator can't be overly unsuccessful, either, or that species wouldn't survive. More of the balance you see every day on the reefs and in the natural world.
Cheers,
Don
