Well, let me just tell you that this won't be very easy, I'm sorry to say.
Firstly, breed a:
Corsola♂ with the move Mirror Coat. Corsola learns Mirror Coat at Level 45.
and
Squirtle/Wartortle/Blastoise♀. The closer to Level 45 it is, the better chance of getting an egg.
This is hard in itself, since Corsola only has a 25% chance of being male, and Squirtle has a measly 12.5% chance of being female. A bit sad, but it's possible. You can go around the GTS.
Wait until you get a Squirtle♂ with Mirror Coat.
The gender matters, but this should be easy since it's an 87.5% chance of getting a male Squirtle.
Now, breed:
The Squirtle you just got before
and
Avalugg/Bergmite♀
The offspring of this Pokemon should be a Bergmite with the Mirror Coat egg move.
Hope I helped. :)
Source: Multiple parts of Serebii
