Step 1: Get a Wailord and a Relicanth.
Step 2: Go to the Sealed Chamber on route 134.
Step 3: Go to the north wall's sign, and use dig.
Step 4:Keep going north, and examine the north wall with Wailord in last position, and relicanth in first.
Step 5: Catch the trio(Regirock, Registeel, and Regice)
Step 6: Go where you caught Regiice while Regice has a nickname and is holding a casteliacone, a never-melt ice, or something else like that.