I think there's a flaw inside the premise of the question. If a unit is powerful in one aspect, such as shooting, it's generally a bad idea to engage it in that manner.
For example, I don't search the Guard codex to find ways to out-melee an Assault Terminator squad... I find tactics to shoot them to ribbons, and learn to suffer losses in order to counter them.
In that regard, I do think your best bet is to go after them with Warscythes and Wraiths. For the [points] expense of the Riptide, I think you'd have a very hard time trying to outgun it. Even engaging it with something cheap, like some Scarabs, will tie it up for a couple of turns, preventing it from shooting.