My Celebi is now at level 74. This is truly how you get it. What you do is defeat the Elite Four. Then trade an Ivysaur (MUST be an Ivysaur) that knows Cut from your Red/Blue game. Go to the shrine in Ilex Forest. Use Cut on the shrine. Then put Lugia at the front of your party ( it doesn't matter what level it is). Use Lugia's Fly to Fly to Mahogany Town. When you have done that, go to the Karate King in Mt. Mortar. Beat him and get Tyrogue. Evolve it into a Hitmontop. Breed Hitmontop with a Ditto and the result will be Tyrogue. Take this one to the shrine (with Lugia at the front remember) and use Cut again. Celebi comes floating out. It is at level 10 when you catch it and it isn't particularly difficult to catch.