Get a male Xatu or Natu in Gen IV, like Heartgold. Have it learn GIga Drain. Then, Poke-Transfer it to Poekmon White. If your Crobat is not a female, breed with a Ditto and get eggs until it is a female. Put the male Natu/Xatu with the female Crobat/evolutions in the daycare. When there's an egg, the Zubat wil know Giga Drain.