Pokémon X and Y Walkthrough, Pokémon Move Sets: Reuniclus
(Please note that the recommendations below are largely made for in-game play. A Reuniclus in a competitive environment will likely make use of different moves to accommodate for smarter, more adaptive opponents. If you have a different build for a Reuniclus, toss it in the comments, please.)
Reuniclus and its pre-evolutions have to be among some of the strangest pokémon on the planet. Are they psychic power incarnate? Gelatinous creatures with visible skeletal structures? Cellular beings that have grown way too large for anyone's good? Tres bizarre indeed. Nevertheless, you'll have a fair powerhouse on your hands if you decide to train a Reuniclus - though you need to handle it properly, or this strange pokémon may wind up as a dud instead.
Reuniclus begins its weird life as Solosis. Level your Solosis to 32 and you'll get a Duosion; go the extra mile to level 41 and you'll have your Reuniclus.
Bug, Dark, Ghost.
Reuniclus is not a well-rounded pokémon, instead focusing largely on extremes. Its HP is extremely ample, and it has a Special Attack stat that will make most other pokémon extremely envious. That said, its defenses are merely average (Special Defense wins over Defense), its Attack is meh, and its Speed is atrocious. The only way Reuniclus will ever go first in a fight is if it's caught up by the effects of Trick Room.
Overcoat: The pokémon is no longer damaged by the effects of inclimate weather, namely sandstorms or hail. It also protects against powder- or spore-based moves, the most prominent of which is Spore. A good ability overall if you know you're going up against a pokémon that enjoys weather changes or some select status ailments (being put to sleep is much less of an issue), but not as good as Reuniclus' other ability.
Magic Guard: The pokémon is immune to indirect forms of damage. This includes bad weather, recoil damage, poisoning, Leech Seed and Curse. You'll get a great deal more use out of this ability, as it effectively renders Reuniclus immune to Toxic users, among other things.
Regenerator: Upon switching out, the pokémon instantly recovers one third of its total HP. Very handy if you like to swap pokémon often, which may come into play if you like to use a support-based Reuniclus (the best kind, in my opinion). Regenerator is a hidden ability.
There are two general roles suited to a Reuniclus. The first is a high-power wall / sweeper. Reuniclus has terrible Speed, but this is instantly remedied if you implement Trick Room. Do this and your Reuniclus is suddenly going first almost every round. Give it Psychic for STAB damage, then follow up with whatever outside moves you need. Shadow Ball, Shadow Ball, Energy Ball and Flash Cannon are all hearty candidates. You can also use Trick Room to unfailingly apply Thunder Wave to your opponents, or forego paralysis and pick up Calm Mind to boost your Reuniclus' killing capacity and Special Defense.
The second role is that of a tank and overall nuisance. Keep Trick Room, but replace the attacks with Reflect and Light Screen to boost your team's defenses. Recover will keep Reuniclus consistently alive, and Toxic will, you know, do its usual thing. Pain Split has a place in this build, as well, if you don't feel like using Trick Room; save it for low-HP moments.
EV / Super Training
Reuniclus already has a monstrous amount of health, and devoting a full portion of EVs to HP will turn Reuniclus into a beefy beast. Beyond this, your focus depends on your moves. If you want an offensive Reuniclus, pop 150 or so EVs into Special Attack and dump the rest into your defenses. If you want a defensive Reuniclus, forget about Special Attack and split your EVs between Defense and Special Defense. Attack is worthless to Reuniclus, and Speed even more so.
Catching a Reuniclus
You can't find a Reuniclus in the wild, but you can track down Solosis, its initial form, in Reflection Cave. It's uncommon, but you should find at least one while trekking through the cave.