Only obtainable in the White version, you can find a Solosis in Routes 16 and 5, while Duosion appears in Route 9.
The line that wins the prize for cutest line in Pokémon Black and White, Solosis is one of the most easily catchable Psychic-types in the White version and one of the Pokémon that was blessed with the fantastic ability Magic Guard. This cute cell isn't only a plush to hug. Although Solosis may start with low stats, Reuniclus can become a mixed defense tank that protects your team while wrecking things with Psychic.
Gym #4 - Elesa (Nimbasa City, Electric-type): Unless you overleveled your team to level 32, you're probably going to be hugging your new Solosis and not planning to use it in this gym. That's a really good idea as Elesa's Emolgas pack Pursuit and are invariably going to use that. They probably won't OHKO your little cell, but it's going to do a lot of damage, not to mention that Solosis is slower than a Squirtle, so use a Ground- or Rock-type for this gym and move on to the next one.
Cheren (Route 5): If you see that Cheren's Tranquill is going to spam Detect and your Solosis knows Reflect, give it a try. If not, let another thing battle the bird. Liepard is a no and the monkeys know Bite, are faster than your cell, and while they die to one Psyshock, flinches are a thing. Pignite is OHKOed by Psyshock, Dewott knows Fury Cutter and can't be oneshotted, and Servine is a possible target, depending on what level your Solosis is, as the snake can set up Growth and be annoying with Leaf Tornado and Mega Drain. If your Solosis doesn't have Magic Guard... only aim for the Pignite.
Gym #5 - Clay (Driftveil City, Ground-type): For this battle, you should have a Duosion, although it can't do a lot here except deal with the Palpitoad. Krokorok is Dark, faster, and packs Crunch, and Excadrill doesn't take much damage from Psyshock. Besides, he likes to flinch everything with Rock Slide. If Clay hasn't used his Excadrill yet, you can try to put a Reflect up on Palpitoad, thus lowering the ace's killing abilities and, if you are lucky and your Duosion has a Hidden Power Grass, OHKO the frog after setting the Reflect.
Bianca (Driftveil City): Herdier has Crunch. Next. Pignite is OHKOed by Psyshock and the monkeys, while still having Bite, can be tanked by your Duosion without problems, Bianca is some levels lower than Clay, so Psyshock the apes. Duosion can battle the Musharna, but unless you have a Ghost/Bug/Dark Hidden Power with 70BP, you aren't going to do any real damage, so let other team member fight it. Dewott still has Fury Cutter, but it's no longer a big deal and Servine with the Magic Guard ability is not going to be a problem to your 125 Special Attack cell.
N (Chargestone Cave): Boldore has really high defenses and can hit hard as well, Duosion can take the risk by using Reflect, but it's better to use another Pokémon. Ferroseed resists Psyshock and has Pin Missile, so unless you have Hidden Power Fire or Fighting, don't try to battle him. Joltik knows Bug Bite, but for this battle your Pokémon should be around 34 and N's are only at 28, so you can Psyshock the spider. Klink resists Psyshock, but can't do much to Duosion either. This battle will totally depend on the moveset of your Duosion.
Gym #6 - Skyla (Mistralton City, Flying-type): Swoobat resists Psyshock and hits really hard with STAB no-item Acrobatics; it knows Assurance, but Acrobatics is worse news. For Unfezant and Swanna put up a Light Screen to lower their STABs power and then hit with Psyshock or let another teammate finish them.
Cheren (Route 7): Super Luck Unfezant hurts, although his only attack is a special one, it's best if you let another one start the battle as it may start the battle with Taunt, preventing you from using Light Screen. Liepard, nope. Pignite is still Pignite, Psyshock and carry on. The monkeys no longer have Bite, but their STAB is around 70-80 with 98 of attack and special attack, they still have laughable defenses and die to one Psyshock, but keep in mind that Duosion has to stand their STAB attack first. Dewott and Servine are as always, if you were able to defeat them in the battle with Bianca, chances are you'll beat them again.
Gym #7 - Brycen (Icirrus City, Ice-type): For this battle you have to teach Flash Cannon to your Duosion or Reuniclus, depending on what level you are and if you deleted Psyshock over Psychic, go to Mistralton to restore it. Vanillish isn't a threat to your Duosion or Reuniclus and his defenses are low, hit it with Flash Cannon and go for the next one. Cryogonal has really high Special Defense, but... its Defense is garbage, this is why you need Psyshock, automatic OHKO. Beartic is strong, Duosion can't do a lot to him, but Reuniclus can fight it as long as you set a Reflect before, is that the case? Flash Cannon until it's dead, then. Otherwise try to one-shoot it or let a Fire/Fighting/Rock/Steel Pokémon finish the battle.
Bianca (Route 8): Stoutland is fast, strong and knows Crunch, monkeys are monkeys and can be OHKOd via Psychic, and for Musharna it's better to leave it to a member that can hit it with super-effective attacks. Emboar can do some damage with Assurance and 120BP Heat Crash (Emboar's weight is almost eight time as much as Reuniclus'), but if you set a Reflect before, a Psychic is an OHKO. Samurott's only STAB move is Aqua Jet which Reuniclus can stand without problems and then 2HKO with Psychic. Serperior knows Coil and has a decent Bulk, Reflect can help a lot here, but if you don't want to worry about your cutie use another Pokémon for this fight.
Gym #8 - Iris/Drayden (Opelucid City, Dragon-type): Reuniclus can try to one-shot Fraxure with Psychic, but it can't do anything to the other two: Druddigon knows Night Slash that can hax in a critical hit and Haxorus can set up one Dragon Dance and kill everything in your team. If your Reuniclus has Thunder Wave you can try to win with an annoyer strategy, but for the sake of your cell, use another Pokémon.
Cheren (Route 10): The monkeys and starters aren't really different from your last battle with Bianca, kill the monkeys and do whatever you want with the others. Unfezant can be a little annoying with Taunt and Detect, but Reuniclus can kill it with Psychic easily. No, Liepard is still a no.
Elite Four Shauntal (Pokémon League, Ghost-type): Your Reuniclus probably knows Shadow Ball, but these ghosts are either bulky enough to stand a non-STAB Shadow Ball or powerful enough to OHKO Reuniclus, let others put an end to these tortured souls.
Elite Four Grimsley (Pokémon League, Dark-type): Two words: Dark-types. And yes, the lead is a Scrafty.
Elite Four Marshal (Pokémon League, Fighting-type): He has four fighting types and no, none of them is a Scrafty, although Throh and Mienshao know Dark- and Bug-type attacks. If your level is high to be more prepared for the last two bosses than for the Elite Four, this battle is going to be just using Psychic and OHKOing everything except the Sturdy Sawk... that goes down the next turn (or the next two turns if Marshal didn't use his healing items).
Elite Four Caitlin (Pokémon League, Psychic-type): For this battle, everything depends on what moves your Reuniclus has. Light Screen is a really good move for this fight as every Pokémon that Caitlin owns only knows special attacks; Shadow Ball is another good choice to hit for super-effective damage. The easiest opponent would be Reuniclus as it is the only one that doesn't come with Shadow Ball, so put a screen and kill it with your own Shadow Ball. Sigilyph can be a real pain, but a Reuniclus behind a Light Screen can't be OHKOed by the totem bird, so shoot some Shadow Balls or a Thunder and see the bird die. Musharna is under control with the Light Screen, but it's still really bulky and with Charge Beam she can buff her Special Attack, go for it only if you are sure that Reuniclus can 2HKO or OHKO it. Gothitelle is the most dangerous of the four as it packs Calm Mind that not only buffs its decent Special Attack, but its Special Defense too, OHKO or change to something with Crunch.
Zekrom (N's Castle): Zekrom is going to be a really hard opponent for the cell as a 150 attack Fusion Bolt hits really hard and Reuniclus doesn't pack anything that is super effective, so it's better to leave the battle to a Ground or a more tanky Pokémon. If you are planning on changing someone for Zekrom, I'm going to be honest, let Reuniclus take a rest. Not joking, these two bosses don't have any Pokémon weak to Psychic and to make matters worse, there are a total of three Dark-types, one Ghost-type and things with Crunch. You can do a lot with Reuniclus, but is preferable to leave him behind than sacrifice a Pokémon that can still be useful in battle.
N (N's Castle): In case you are sticking with the plushy blob until the end, here is the strategy to make the cell survive: N's dragon has a higher level than yours, if you aren't using Zekrom, Reuniclus can be a good lead for the battle as it can take damage from Fusion Flare without problems and put a Screen and then recover its health. Psychic isn't going to do a lot of damage as Reshiram has a really high Special Defense, but it can be beatable. Vanilluxe is a joke for Reuniclus, Hail can't damage the cell no matter which ability it has and the ice cream attacks aren't going to kill it. Don't try against the Kinklangs as one of them is Zoroark, if you beat the fox earlier and you have Focus Blast in your Reuniclus, go for it. The fossils have Crunch, the two can 2HKO Reuniclus without Reflect and at least the turtle is definitely going to survive one hit.
Ghetsis (N's Castle): The Cofagrigus loves to Toxic stall, but it has a good bulk and Shadow Ball, so it isn't worth the risk. Bouffalant has high Attack and can use Head Charge with 120BP, if Reuniclus survives it can finish him with Psychic, but it's better to use other Pokémon. Seismitoad can be dealt with Psychic and a potion and, ironically, the Hydreigon can be battled by the cell as long as it has Focus Blast and Light Screen, use the second one first to take less damage and then aim for the Focus Blast. Eelektross knows Crunch and does some notable damage. NOPE for the Bisharp.
Post-Game: It's tougher than before. The rematch with Marshal and Caitlin still makes Reuniclus see some usage, but the rest is unkind to it.
While some of the best TMs for Reuniclus are in the postgame, the little cell still has a really good moveset with a lot of options to choose from. A freshly captured Solosis can come with Reflect, Light Screen, Hidden Power, and other moves such as Rollout, Psywave and Charm, with the Screens being really powerful for early and mid-game. At level 24 it learns Recover and at the next one Psyshock, an 80BP special attack that hits the enemy Defense and can deal with Pokémon like Cryogonal. Endeavor at 28 is an interesting move as Solosis is a really slow Pokémon, so the damage is guaranteed. The big thing comes at 39 with Psychic, here you can decide between Psyshock or Psychic, but except for Cryogonal, most of your targets are going to have better Defense. Other moves that this line can learn via level up would be Future Sight, Pain Split, Skill Swap, Heal Block and Wonder Room, some of them are interesting, but there are better options.
As for TMs you have some good coverage in Thunder, Shadow Ball, Flash Cannon and Focus Blast. Thunder is risky because of its low accuracy, but is a powerful move that goes really well with Reuniclus' 125 Special Attack, Shadow Ball is a classic for Psychic-types, Flash Cannon is rare in Psychics that aren't also Steel type and a good option for Ice- and Rock-types, if you are planning on fighting Dark-types you can hit (or in many cases, miss) them with Focus Blast, which is another classic for Fighting-type coverage. Other options are Thunder Wave for paralysis and Grass Knot for more coverage, but only Gigalith can really suffer from this attack and the rock has Sturdy as its ability.
Fighting-types: Unova is known for having lot's of Dark types, Solosis main weakness through the adventure. That's why some strong Fighting types will deal with that pesky and annoying Dark types and even resist another of Solosis's weakness, Bug, able to deal with them thanks to Rock type moves like Rock Slide or Stone Edge.
Dark-types: They resist two of Solosis' weakness, Dark and Ghost, and hit the second one super effective, making them some good addition to the team. The best Dark types would be the ones that aren't weak to Bug or can hit Dark types with effective moves.
A few examples of this include: Scrafty, Bisharp and Mandibuzz.
Steel-types: They resist everything that hurts Solosis and Solosis resist and hits the Fighting types that hurt them. Some of them can hit Solosis's weakness with coverage or STAB and have normally high Defense that can be paired with Reuniclus high Special Defense for a better defensive core.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Solosis Line Ratings
What Nature do I want? Modest is the best option if you want a full offensive Reuniclus that can OHKO half of the things it encounters. For a more defensive option, Bold can help your cell’s low Defense for tanking better the physical attacks and Calm to rise the Special Defense, you can sacrifice Speed using Relaxed (+Def), Quiet (+SpAtk) or Sassy (+SpDef) natures as Reuniclus has a base speed of 30 and most of the Pokémon in Unova are going to outspeed it anyway.
Which Ability do I want? Magic Guard is known for been one of the best abilities in the game, this ability nulls all of the indirect damage the little cell can suffer, that means Poison, Burn, Leech Seed, Hail, Sand Storm, Spikes, Stealth Rock, recoil move or Curse from a Ghost Pokémon aren’t going to do anything (you can still be Leech Seeded or Poisoned even if it can’t harm it), it can even null the damage from Life Orb and get the bonus power from it. Overcoat can protect from Hail and Sandstorm… at least isn’t a negative ability.
At what point in the game should I be evolved? The Solosis line learns the level up moves at the same level, so you can evolve Solosis into Duosion and this one into Reuniclus as soon as they can.
How good is the Solosis line in a Nuzlocke? It isn’t one of the best choices for a White run. It has access to one of the best abilities in the game, a really good moveset with great coverage, early screens and a 80BP attack at low level and a decent bulk with a high Special Attack, but Unova is full of Dark types and things with Crunch and Night Slash that outspeed this cute little Pokémon. Psychic-types have a hard time here and only one important trainer specializes in a type they have an advantage over, so yes, while Reuniclus is a really good Pokémon for a White run and you still want a Psychic type in your team, a Musharna is a better option (if you are lucky enough to have a Munna as your first encounter) as it has a more bulky stats with a higher chances of surviving in this dark region.