Unlike other Mythical Pokémon in earlier routes, it will not just appear in front of you. You’ll have to lure it out from behind some crystals.
To bait it out, you have to do the following:
- Take the alternate route to the crystal cave provided in rank 2.
- Throw Illumina Orbs at the Mawile and two Carbinks that are standing together at the entrance of the crystal cave.
Diancie will come out from behind the crystals to the right of you . You can throw Illumina Orbs at her to get her to do some fancy animations.
Bringing out Diancie will also make the Mawile sitting in the middle of the crystal room turn around, unlocking another alternate route. This secret route has a new Vivillon color, as well as Jolteon, so make sure to take that route if you want to complete your Photodex. This route requires you to lure out Diancie whenever you want to take it. It does not stay unlocked after you clear the level.