I found this out and i couldn't believe that i saw Celebi. Go to the place where you open the shadow pokemons heart! The use time flute. Be careful because there are only three time flutes. The time flute brings celebi to open the pokemons heart.

When you us a shadow pokemon in a battle the black line where the exp should be goes down. But did you know it goes down when you walk around. And you can us sents to make it go down to!

Hope this helps!