You will find it in the Exit Cave using Illuminate Orbs.
To get to this you will have to go through the alternative route to the open crystal cave on Rank 2> at the entrance you will launch Illuminate Orbs to Mawile and two Carbinks that are in the same place to get Diancie's attention behind the crystal, and if you throw more orbs of light it will make different animations or poses.
Also, the Mawile in the center in the crystal room will turn around to open the alternate route. Which will be canceled after having exceeded the level.
After reading this guide on where to find Diancie in New Pokemon Snap, we hope that you manage to find this Mystical Pokémon and pass this level.