to obtain mega Lucario. First,you beat

Korina. Then go to the mega evolution 

tower. Go to the very top, and you will see

Korina. Lucario will have a very strong sense

with your character. He will want to battle aside

your character. Then,Korina will give you a Mega Ring.

Then,you battle her. Lucario is holding the Lucarionite.

It doesn't matter if you lose or not. She will give you

Lucario holding the Lucarionite.