Clefable is just getting unfair... If its at full hp when its sent out, theres absolutely nothing you can do.
http://replay.pokemonshowdown.com/ou-89315092
Another guy (the same guy?) said basically this exact thing in the XY simple answers thread, so I'll C+P my answer here too. Before that, I'll just say you lost this match because of your own misplay on turn 30. You leech seeded rather than just going for the sludge bomb. You can't argue Clefable is un-killable if the only reason you don't kill it is because of your own misplay. After that point, quite frankly the only reason you didn't win despite the misplay was because you got REALLY unlucky with not getting a crit despite hitting so many times. Finally, it was clearly max special defense, which means it would have been killable if you had a physical attacker like the ones below.
C+P from other thread:
List of pokemon that can (usually) counter Clefable--that is, switch in on Clefable at full HP and threaten it out or kill it--assuming you don't waste turns letting it set up:
Swords Dance Aegislash (esp. with Iron Head)
Mega Venusaur (sleep powder is a plus)
Bisharp with at least 104 HP Evs
Heatran (can tank all day thanks to quad-resisting moonblast, being immune to flamethrower, and resisting stored power, and can roar it out to give a different pokemon a better chance to kill it or spam flash cannon until it crits)
Swords Dance Talonflame
Excadrill (even Jolly) - (resists moonblast and stored power and can flinch with iron head - AV helps with flamethrower)
Mega Mawile
Landorus-I with sludge wave
Other options:
Taunt it
Trick it a choice item (esp. if it's the last thing left on your opponents team so it can't switch out)
Put it to sleep and kill it before it wakes up (Mega Venusaur is best for this)
pHaze it out and try again later
In your case you could have saved Ditto and switched it in once Clefable set up