The problem with Clefable, is not that it is unstoppable. No the problem is that Clefable is so versatile, ther eis no 100% counter for it. Yeah Fighting types are your best bet but look at the fighting types that can actually take out clefable:
 
Hitmonlee
Hitmonchan
Hitmontop
Toxicroak
 
If there are any other fighting types in UU there will most likely not ko. And don't forget clefable takes no damage from leech seed, stealth rock, spikes, poison, ect. Almost a perfect counter to all of the uu pokemon that rely on that to take an opponent out. A Simple Ice Beam/Calm Mind/Softboiled/Filler set could take out threats such as Rotom, Jumpluff, ect. quite easily. But then you also have Choice Band, Specs, Belly Drum, Cosmic Power, SleepTalk, Life Orb, Defensive Barriers, Wish Support, ect. It gives no solid way to counter Clefable.