Clefable can run a multitude of different sets, but generally speaking, there are two main sets.
First, the Magic Guard Cleric. Y'know, Wish/Aromatherapy/etc.etc.
Second, there's the bulky setup one. Unaware Cosmic Power + Stored Power is a difficult thing to get around, since you can't boost your offense to match the target's defense. As long as you've eliminated the possible phazing threats, it's just a matter of setting up and grinding your opponent's team down.
Currently playing: Pokemon Y (Name: Ensei), Xenoblade Chronicles, 3DS FC: 2277-7562-5299 FS: Ghost @ Drfiblim, Lampent, Pumpkaboo MegaGross! :D