When you have the Celebi in your party, and you're at the Ilex Forest, follow this guide:
GUIDE:
Step 1. Click the small house in the middle of the Ilex Forest. Celebi will react to it and take you along with Lyra back in time.
Step 2. Once you are back in time, you will see a short dialogue of your rival and his father, leader of Team Rocket.
Step 3. After the dialogue, Celebi will take you to a mountain where your rivals father is. You must defeat him and his level 40 - 45 Pokemon in a battle.
Step 4. After defeating him, he will say that you foiled his plan this time just like how another kid foiled it 3 years ago. Then he will leave.
Step 5. At this moment, Celebi will transport you back to the Ilex Forest, in your time.
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Celebi
Go to the Ilex Forest Shrine of Celebi and tamper with it. You must have Celebi walking with you. It will take you back in time and you will have to face the leader of team Rocket.
go to ilex forest and interact with the shrine while walking with celebi and it will send you back in time 3 years ago.
You get Celebi from an event or trade, then you can walk up to it and Celebi will take you back in time.Be sure to have at least four strong Pokemon (over level 40)
You must get the Pikachu-colored Pichu and transfer it to HG. You will get the Spiky-eared Pichu if you go to the Celebi Shrine in Ilex Forest.