Things You'll Need
Instructions
To obtain the Celebi, you first must beat the Elite Four. Then, capture a sneasel. You can use an ultra ball to capture the sneasel immediately. Aggressively level the sneasel up to be level 57. At that time, the sneasel will learn a move called "beat up." Be sure that you place the move into the third slot out of the four it knows. That step is crucial.
Travel to Goldenrod, in the northern part of the city. You need to find the pokemon name changer. Just follow the main (middle) road, and at the very top, it will be the first house on the left. Change the name of your sneasel to cccccelebi. You need the four extra C's for later use. Travel to the south of Goldenrod City on Route 33 to find the Pokemon day care.
Step into the day-care center. Save your game and then go to the PC box. Deposit the sneasel into an empty box within the PC. You will now make a clone it. This is the part that may corrupt your file. Change the box that the sneasel is in to another empty box. While it's saving, turn the Game Boy off when the box hits the T. The dialog box should say: (SAVING... DON'T TURN OFF T). You must do this for four clones.
Create a bad clone. To get a bad clone, turn off the power at any other different time. The Pokemon should be at level zero, and the information on it will be a bunch of trash symbols. Withdraw the clone and give it to the day care person. Take the clone back, and it should have a garbage can icon over it. It will cost 100 pokemon dollars. Place the clone into the first slot of your party. Then go back to the PC. In the PC box, the game will read the glitch pokemon and stop trying to read any of the others. Place an expendable pokemon into the first slot of your party now. The clone should now be second in your party. Now go back to the day-care person and give the glitched pokemon back.
Going back to the PC box, deposit your sneasels and your expendable pokemon into the box. You should notice that the celebi's names are losing the C's until there is just one. Withdraw the expendable pokemon and the sneasel with only one C in Celebi. Go back to the day-care person and retrieve the glitched pokemon from him. This will change the levels of the sneasel. Give the sneasel to the day care and then get it right back from them. If all goes right, you will have Celebi.