(I'm not sure if I'm actually allowed to post two, but oh well :P)
Ground Counter
Steelix @ Steelixite
Ability: Sand Force
EVs:252 Atk / 252 SpD / 4 HP
Sassy Nature
- Heavy Slam
- Roar
- Earthquake
- Stealth Rock
Mega Steelix, while not the most popular and obvious answer, can do quite well against this particular Clefable. Immunity to Thunder Wave and resistance to Moonblast, (SpD investment and Sassy nature is not needed, but it turns unboosted Moonblast from a 3HKO into a 7HKO, making Mega Steelix safe to switch in to an oncoming Moonblast).
0 SpA Clefable Moonblast vs. 4 HP / 252 SpD Mega Steelix: 40-48 (13.6 - 16.4%) -- possible 7HKO
A Heavy Slam with max investment has a guaranteed to OHKO Clefable, which is necessary before it falls victim to that blob.
252 Atk Mega Steelix Heavy Slam (120 BP) vs. 252 HP / 160 Def Clefable: 408-480 (103.5 - 121.8%) -- guaranteed OHKO
However, if Clefable sets up too many Calm Minds, this is where Roar comes in.
+3 0 SpA Clefable Moonblast vs. 0 HP / 252 SpD Mega Steelix: 100-118 (34.3 - 40.5%) -- guaranteed 3HKO
Should Clefable reach +3 or higher (+4 is also a 3HKO, but +5 and +6 is a 2HKO), it is a better idea to Roar then allow it to set up more or hurt Mega Steelix bad. If the Clefable switches back in, without the boosts it had it is more vulnerable, and you are free to Heavy Slam away.
Earthquake and Stealth Rock are filler moves. Earthquake has a low chance to 2HKO and Heavy Slam should be used over it.
UPDATE: Keep in mind that Mega Steelix
DOES NOT outspeed Clefable, but you can speed creep it with max Speed investment.
And an extra one :P:
Electric/Flying Check
Thundurus @ Leftovers
Ability: Prankster
EVs: 252 SpA / 252 SpD / 4 HP
Calm Nature
- Taunt
- Sludge Wave/Flash Cannon
- Thunder Wave/U-Turn
- Thunderbolt
And I thought Clefable was unbeatable...
Thundurus, while not being to reliably counter Clefable like Mega Steelix, shuts down it setting up and reduces the threat that it faces. Like Mega Steelix, it has an immunity to Thunder Wave, but unlike Mega Steelix, it outspeeds even without Prankster. SpD investment and Calm Nature means that an unboosted Moonblast is a 4HKO.
0 SpA Clefable Moonblast vs. 4 HP / 252+ SpD Thundurus: 82-97 (27.3 - 32.3%) -- 63.7% chance to 4HKO after Leftovers recovery
The most important move for Thundurus will be Taunt. Giving priority to Taunt with Prankster means that Clefable will be unable to use Calm Mind (or Thunder Wave, so Thundurus can switch out into something that can kill it but doesn't want to get paralyzed, and also Soft-Boiled, which can be a problem for Thundurus). So, the only thing Clefable can do is Moonblast. In this situation (and without hax), Thundurus would win every time.
Should Thundurus and Clefable be in a one-on-one situation, Thundurus will either have Sludge Wave or Flash Cannon. Sludge Wave covers for Clefable better (although is hax does happen, Clefable won't be affected by poison), while Flash Cannon covers for Thundurus' own weaknesses better. It doesn't matter which one it has though, as both will 2HKO (but it's not guaranteed with Flash Cannon) an unboosted Clefable (however, if Clefable does set up a Calm Mind, it will be a 3HKO).
252 SpA Thundurus Sludge Wave vs. 0 HP / 96 SpD Clefable: 200-236 (60.4 - 71.2%) -- guaranteed 2HKO after Leftovers recovery
252 SpA Thundurus Flash Cannon vs. 0 HP / 96 SpD Clefable: 168-198 (50.7 - 59.8%) -- 85.9% chance to 2HKO after Leftovers recovery
Like with Mega Steelix, the last two moves are fillers, although U-Turn can be useful after using Taunt to switch out.