you need the shiny pichu from the gamestop event. go to the ilex forest shrine with pichu in your party. press A by the shrine and the spiky eared pichu will pop up and join your team if you have an open spot in your party.
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 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.
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.
You can get a Spiky-eared Pichu in Pokémon HeartGold and SoulSilver if you get the movie-event Pikachu-colored Pichu and bring it to Celebi's Shrine in Ilex Forest in HeartGold or SoulSilver.
ilex forest shrine
Go to a shrine in Ilex forest and after that you time travel
The note in the Ilex forest is towards the beginning of the forest. It is next to the Celebi shrine. You will most likely need cut to access the note and shrine, though.
To get the Spiky-Eared Pichu you have to bring the special shiny Pichu from a Gamestop event to the Ilex Forest Shrine.
Go to the shrine on ilex forest. Talk to it!
take the GS ball to the shrine in ilex forest
In Ilex Forest and upwards where the tree is blocking the way where you need to use Cut.
Celebi is found in the Ilex forest near the 2nd Gym, at the shrine
You need to get the event Celebi and then go to the shrine in Ilex Forest.
you need the shiny pichu from the gamestop event. go to the ilex forest shrine with pichu in your party. press A by the shrine and the spiky eared pichu will pop up and join your team if you have an open spot in your party.
You must have the pichu from gamestop, and take it to the shrine in the ilex forest.
You need the special Celebi which is obtained from Mystery Gift and only Mystery Gift.