If you have not already been to Abyssal Ruins, I recommend you go there as soon as possible. Abyssal Ruins is a underwater temple located in Undella Bay, it can be accessed through the use of HM06 Dive. Most of Arceus's Plates are located here. There are four floors and no wild Pokémon or Trainers. The Ruins are filled with 32 Relic items which have a total value of 1,706,000 Pokémon Dollars when sold to the billionaire item maniac.
Amulet Coin/ Luck Incense
Have your Pokémon hold the Amulet Coin/ Luck Incense and defeat the Elite 4.
Hope I helped :)