Instructions
Go to Pokemon Square. There will be an event that notifies you of the opening of Luminous Cave its availability for exploration.
Travel to Whiscash Pond by yourself, which is north of Pokemon Square. When you try to enter the area, your partner will ask if he can come with you. Choose no to travel by yourself.
Select only the Pokemon you want to evolve to travel with you. Every Pokemon will be evolved in the group, so only bring the two Pokemon who are friends.
Travel to Luminous Cave in Whiscash Pond. The cave is located near the north-east section of the map. When you enter the area, a crystal will be near the entrance.
Talk to the crystal by pressing "A." The crystal will ask if you would like to upgrade the two Pokemon who are friends, as long as they meet the requirements. Each Pokemon has an individual level that must be met for it to be able to evolve. Also, some rare Pokemon require special items be presented to the crystal to evolve. The crystal will inform you of the items or levels needed to evolve the Pokemon.
Click "Yes" on the evolve prompt to evolve both Pokemon. The two will continue to be friends, even in their new forms.
Return to town and gather the rest of your Pokemon.