Welcome to Gaia! ::

Its a random chance that you'll get it this way but here's how I got mine:

1. Get Mystery Gift

2. Go through Mystery Gift until you get a pokeball called: TERU SAMA

3. get every pokemon but Celebi and then go see Prof. Oak

4. Then go see Prof. Elm (Must have shown Togepi before hand or else it doesn't work! and I do mean before you get mystery gift)

5. Go See Kurt.

6. Go through Ilex Forest and place the item Kurt gives you in the shrine.

7.Go back through the forest and look for hte ledge formation.

8.(Must have Teru Sama with you) Have your Starter pokemon in the first slot then have the first pokemon you caught in th esecond then the second pokemon you caught in the third yada yada... You should see a shining ball. approach it and then go to your menu then use the teru Sama on it DON"T PRESS A NEXT TO THE SHINY THING

9.You will see a giant round shining thing press B

10. Take a step back and before you exit the forest Celebi will battle you at level 75.

Celebi for people that took Chikorita:

Moves:
Psychic
Leech Seed
Head Butt
Perish Song

Celebi for Cyndaquil:
Psychic
Perish Song
Psywave
Heal Bell

Celebi for Totodile:
Psychic
Leech Seed
Perish Song
Mega Drain

Wow, i never got that item in mystery gift...*cries*
Ah well.