Oh, I have 5 assassins in a 20g with cherry shrimp. I forget to drop snails in there for them sometimes, and they'll take down a shrimp. If I want to watch them eat a snail, I just go grab a medium sized ramshorn and toss it into the tank. They come running(?) from each direction and descend like hungry vultures. The poor ramshorn doesn't have a chance. It's funny to because there's two dominant snails and they'll push their way right into the ramshorn, shoving the others away.
But yeah, when they took down my huge trapdoor snails it was all of them on one, and they didn't even finish their meals, they left the bodies half-eaten inside of the shells and the resulting quickly-fouled water destroyed my RCS colony.