Increase Friendship with Your Diancie
Building Friendship
Friendship increases through regular interaction and positive experiences. Keep Diancie in your active team and use it in battles. Each victory, especially against tough opponents, strengthens your bond. Avoid letting Diancie faint repeatedly, as this damages Friendship.
Daily Bonding
Spend time with Diancie by having it follow you as you explore and complete tasks. The more time Diancie spends active in your team rather than in storage, the faster Friendship grows. Consistent use is key—sporadic battles won't build Friendship as quickly.
Healing and Care
Heal Diancie promptly when it takes damage. Using Potions or visiting a Pokémon Center shows you care for its wellbeing. Avoid unnecessary fainting, and don't use it in situations where it will repeatedly lose.
Checking Progress
While the mod doesn't show explicit Friendship numbers, you'll notice improved animations and behaviour when Friendship is high. Once you've consistently battled and trained with Diancie for a reasonable period, attempt the Mega Evolution with a Nether Star to see if Friendship is sufficient.