There is an area behind Alder's House. There is a large rock with cuts in it. Interact with it and you can teach Keldeo Secret Sword, which will cause it to turn into its Resolute Form. As long as Keldeo has Secret Sword it will remain in Resolute Form.
*A competitive note: Had to put this somewhere where I can link people to instead of repeating myself on chat.*
You don't want Keldeo to be Resolute Form. Keep it in its standard form. Having it in Resolute Form will be an instant indication that it knows Secret Sword. Have it in standard form and keep your foe guessing whether you have Secret Sword, Focus Blast, or even a Physical set. While a small advantage, every little bit helps!