You could try to give it a damage-boosting item like an Expertbelt, Wise Glasses or any item that boosts Psychic moves.
Try to use Cosmic Power once before attacking with Stored Power as this will boost the damage of the move Stored Power.
If you dont have a damage item and it still doesnt work, try to level-up your Clefairy even higher. You can use the Golbats in the castle for that, they respawn every 5 minutes.