The ultimate answer to Clefable and Ditto is the little monster known as tyrogue. Both are weak to him. With a pluspower and a hit, Clefable falls. With a hit (and 30 HP) Ditto falls.
Another annoying way to get rid of ditto is Mysterious fossil. You discard it, but you do not get the prize.
Ditto is very prone to getting gusted back to the bench, hit for 30 damage, then faced with a baby, or many other natural weaknesses of the creature. If you really want to stall against a ditto, throw your own ditto up at them, or a Clefable. Watch them just groan. Elekid works well too, or Igglybuff, in that they have no attacks to copy!
M45