Crobat can learn Nasty Plots by following these simple steps:
- Get a male Honchkrow with Nasty Plot, it gets the move at level 35
- Breed it with a female Zubat/Golbat/Crobat and get the egg
- Hatch the egg and it should know Nasty Plot
Sadly though, Crobat can learn Giga Drain only if you teach it the move via move tutor in Pokemon Black 2 / White 2. But now in X/Y Giga drain is an egg move, here are the Pokemon you should breed it with to get the move:
Zubat, Golbat, Crobat, Natu, Xatu, Woobat or Swoobat that already know the move; however none of them learn it naturally, there are only two ways to get that move into any of the mentioned Pokemon:
- Make them learn it via TM in generations 3/4
- Make them learn it via move tutor in generation 5
And then transfer them to your X/Y game, breed and then you can get a Crobat with Giga Drain.
So if you don't have the game, you cannot get it yourself, only ask others to who have the game to do it for you.
Hope I helped!