Cradily can only get Giga Drain when you get its pre-evolution, Lileep, from the Dream World. It is IMPOSSIBLE to get through breeding. It is a Dream World Exclusive move.
You can get one at the Dream World area, Sparkling Sea, but you need 7500 Dream Points to unlock him.
Source for move info: Bulbapedia- Lileep
Source for it's location in the Dream World: Bulbapedia-Dream World/Sparkling Sea