Catch Cosmog After Completing the Storyline
Once you have completed the main story, you will be able to obtain Cosmog.
【Where to Obtain Cosmog】
Location: Altar of the Sunne / Altar of the Moone
After Completing the main storyline, have Solgaleo or Lunala in your inventory, and head towards Altar of the Sunne / Altar of the Moone during the designated time. At the altar, you will find a portal. Enter the portal and you will be transported to Altar of the Moone/ Altar of the Sunne. (This is depended on the version of your game.)
Then go to Lake of the Sunne / Lake of the Moone, and make your way towards the top of the platform. You will then enter a cutscene and after watching it, you will be able to obtain Cosmog.
Obtain a Second Legendary Pokemon
Cosmog will evolve into Cosmoem at Lv.43. And once Cosmoem reaches Lv.53, it will evolve into Solgaleo if your version is Sun, and Lunala if your version is Moon.
Since you will have 2 of the same legendary Pokemon, it would be best to trade one of them with a player that has the version you don’t.
Is it Possible to Pick the Perfect Individual?
Synchronize was effective.
After several trials, Cosmog always had the same Nature as the Pokemon with Synchronize. The chance of Synchronize succeeding might be set to 100%. Also, it will always have a V for its Individual Value.
Therefore, it is possible to pick the perfect individual.
If a Pokemon with Synchronize is leading the party, any wild Pokemon that you encounter has a 50% chance of having the same Nature as the Pokemon with Synchronize.