Get a Sharpedo or two. Gyarados could do too, but Sharpedo is better because it's got higher sp. atk. Crunch & Surf will do exceptionally well, and try to target one Pokemon instead of hitting both at the same time. Also, take out their Xatu before it can spam too many Calm Minds.
If you need to gain some extra levels, go to Pacific Log Town and train there by fishing for Sharpedos and Wailmer.