Sableye can be found as a random encounter. On the rightmost cavern, after obtaining the purple crystal key, interact with the purple crystal to gain access to the exit. The cavern north with the prompt "The crystals' light flickers dimly" means that it must be turned into a Dark Crystal Cavern field via usage of Dark Void, Dark Pulse, Light that Burns the Sky, or Night Daze in-battle.
A telecominicator is located in entrance of the end cavern.
Hey! I see you're coming in from Iolia Valley. Believe me, getting this telecom to work all the way out there was not easy.
Luna loves to fight in her Dark Crystal Caverns. The gems there warp light energy and make it behave in odd ways. In those caves, her darkness-based attacks will be stronger, but that's not all. Because of the crystals, light attacks will be stronger too! See if you have any that you can use against her.
If all else fails, there is a way to break the dark energy. All you need is a source of unlimited light. That should be enough of a clue, right?