Crobat can learn Nasty Plots by following these simple steps:
> 1. Get a male Honchkrow with Nasty Plot, it gets the move at level 35
2. Breed it with a female Zubat/Golbat/Crobat and get the egg
3. 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:
>1. Make them learn it via TM in generations 3/4
2. 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!