I don't have the best of natures, but the overall balance and max defense/+ nature is leading mine to be a thorn in Gyarados side's of sorts. I am going with the approach to use moves that might get the job done which Cresselia lacks.
In this case, Thunderbolt, Stealth Rock, and possibly Yawn (or Toxic one of the Screens as filler). With the way I plan to EVs, it should be enough to stand up to 2-3 DDs over several hits and 2KO on the typical variants of ideal-stats Gyarados, and enough to outspeed 233-Speed before DD (with a large portion of EVs devoted into a HP stat divisible by 16 HP mod 1).