that's what i'm thinking too: distracting shot on their lead attack and they're done for the next 20 seconds. yes, assassins have some pretty good hexes, but they're nowhere near as deadly as those chain combos.
What i'm trying to say is, any skill that breaks their chain, shuts them down for the time being. (Usually.) Plague Touch is also a great anti-sin skill because when you throw their conditions back on them, they usually panic and find a new target.