Taunt Keldeo...
Its amazing! All those Gastrodons that want to switchin on you to get a Storm Drain boost are met with Taunt, so they can't do anything while +1 Secret Sword 2HKOs.
Providing you have the right team support, I don't think that Keldeo should rely on Hidden Power. Its STABs hit everything important as is, and Taunt gives it lots of room to set up. Like having Spore on Breloom....
:O