i want to trade it to my Pokemon black 2
You have to breed it with a ditto or a Female Empoleon.But I'm guessing you dont have Female Empoleon.
Breed with Ditto
I've done so PLENTY of times...
Okay step by step:
1. Breed Empoleon with Ditto
2. Migrate Piplup to Black
3. Trade to Black 2
Breed the Empoleon with a ditto!