Cosplay Pikachu is available as a gift from an NPC in the Contest Spectacular after the player participates in their first Contest, regardless of result. It has five different formes that can be changed at the counter: Rock Star (Cool), Belle (Beautiful), Pop Star (Cute), Ph. D (Clever), and Libre (Tough).
One of the most recognizable faces in the history of video games, Pikachu makes a roaring comeback in ORAS as Cosplay. Cosplay Pikachu has access to five different moves that ordinary Pikachu does not, although it can only choose one move at a time. By far Cosplay Pikachu's best forme is Belle, which gives it access to a highly coveted coverage move in Icicle Crash. Cosplay Pikachu's great Electric/Ice/Grass coverage, combined with the Light Ball, allow it to tear through a good portion of Hoenn with attacking stats which, when properly invested, rival those of Rayquaza. It should be noted right off the bat, however, that the Light Ball item doubles the current physical and special attacking stats of Pikachu, NOT the base stats. This means that Super Training is very important for Cosplay Pikachu to fully display its strength. Unfortunately, Pikachu's defenses are worse than a wet paper bag, so yeah. Thankfully to Pikachu's impressive base 90 Speed, it can serve as an excellent lead/Volt Switcher.
Rival (Route 110): Wailmer easily falls to Electro Ball in a hit or two; keep in mind that it does have Rollout, but you can definitely KO it before the attack starts dealing too much damage. Shroomish can annoy you with Leech Seed and Headbutt, but if you are running Pikachu Belle (which you absolutely should), then Icicle Crash can take care of it. Slugma, due to its low speed, takes a great deal of damage from Electro Ball, and Pikachu should be able to tank at least one Incinerate. As for the starters, it is best to stay away from all of them: Combusken and Marshtop will likely kill our electric mouse with Double Kick and Mud Bomb, respectively, whereas Grovyle will harass you to death with a combination of Mega Drain and Pursuit.
Wally (Mauville City): This battle is not really hard. Ralts only has Magical Leaf and Disarming Voice to hit, so Pikachu can outdamage it easily.
Gym #3 - Wattson (Mauville City, Electric-type): Thanks to its hidden ability Lightning Rod, Cosplay Pikachu fares incredibly in this fight. His lead, Magnemite, can only damage you with Tackle, so you are free to spam Rock Smash, Flying Press or Electro Ball until you net the KO. (It should be noted that due to Flying Press's wonky mechanics, it only deals neutral damage to Steel-types despite being a "Fighting" attack.) Voltorb can only damage you with Rollout, which can become rather threatening after a few turns; however, thanks to Voltorb's paltry base 30 Attack, you should have plenty of time to switch to a teammate. Magneton can only damage Pikachu with Magnet Bomb, which works off of its much weaker attacking stat; however, it can annoy Pikachu with Supersonic, and Pikachu will struggle to deal a significant amount of damage in return.
Tabitha/Shelly (Meteor Falls, tag battle with rival): Regardless of which version you're playing, Pikachu really does not want to be in this battle for very long. In OR, Tabitha's Mightyena can chip away with Snarl and harass with Swagger, and his teammate's Koffing will easily OHKO Pikachu with Self-Destruct. In AS, the Mightyena is the same, and the Grimer has Mud Bomb. Just Volt Switch out of there and let a teammate handle this fight.
Tabitha/Shelly (Mt. Chimney): Pikachu needs to avoid the Poison-type due to Self-Destruct and Mud Bomb respectively, and Numel too, because it has Magnitude. However, it can easily Thunderbolt the Carvanha into oblivion, providing it survives its Aqua Jet.
Maxie/Archie (Mt. Chimney): The best thing you can do against the Mightyena is Volt Switch out of the way; Pikachu will likely get OHKO'd by a critical STAB attack. You should be able to handle the Golbat just fine, but be wary of its Confuse Ray. Avoid the Camerupt at all costs for obvious reasons; Sharpedo, too, may in fact OHKO with STAB Assurance, so just Volt Switch out as usual.
Gym #4 - Flannery (Larvaridge Town, Fire-type): Thanks to Pikachu's terrible defenses, it actually has a chance to get OHKO'd by her lead Slugma's Overheat, so once again, simply Volt Switch out. Numel is part Ground, so avoid that matchup. Torkoal is immensely bulky and will flatten our rodent instantly, so simply Volt Switch out.
Gym #5 - Norman (Petalburg City, Normal-type): Using Pikachu in this fight is a great way to get it killed. Seriously. Really the only thing that you could potentially do is Volt Switch out of the Vigoroth, but whatever you do, do NOT let Pikachu get hit. It will die, and Flying Press does not deal enough damage.
Courtney and Matt (Southern Island, tag battle with Steven): Unless Pikachu's teammate is fast and can OHKO Camerupt, this isn't a place for Pikachu to be. However, if you have that, then you can take on Sharpedo with Thunderbolt.
Tabitha/Shelly (Weather Institute): Stay away from Tabitha's Camerupt for obvious reasons. Shelly has only a Sharpedo, so Thunderbolt away.
Rival (Route 119): Once again, Slugma, Shroomish and Wailmer can all potentially be 2HKO'd by the appropriate attack; however, keep in mind that they all now have decently strong STAB attacks which will do about half to Pikachu, if not more. All three starters should be avoided like the plague.
Gym #6 - Winona (Fortree City, Flying-type): Unfortunately, this fight comes right before you get access to the Light Ball; nonetheless, Pikachu can pull its weight and then some. Discharge should take care of Swellow, Pelipper and Skarmory without too much trouble. Just be wary of Swellow's Wing Attack damage output despite resisting. Skarmory does not have Sturdy. Although you might feel tempted to click Icicle Crash against Altaria, it really is not a good idea; even if you max out your Attack, then it still probably won't OHKO. Altaria, meanwhile, will destroy Pikachu with Earthquake. Just Volt Switch out.
Courtney/Matt (Mt. Pyre): The exact same as the Weather Institute fight, except Pikachu should be holding a Light Ball now. Nevertheless, Camerupt is still not viable. You will be safer against Sharpedo, though.
Rival (Lilycove City): A Light Ball-wielding Cosplay Pikachu does extremely well in this fight. Discharge destroys Swellow and will probably OHKO Wailord too unless you have a crazy low amount of Special Attack EVs. Pikachu can easily 2HKO Magcargo with Thunderbolt, whereas Magcargo will be unable to 2HKO in return even with a critical hit. Swampert easily falls to a Grass Knot. Blaziken can OHKO you with Blaze Kick, so just Volt Switch out. Both Sceptile and Breloom fall into the "special case" category: if your Pikachu has the maximum amount of Attack possible, then it is possible for Icicle Crash to OHKO these Grass-types; however, nine times out of ten, you are going to want to simply Volt Switch out of there lest Pikachu get OHKO'd in return.
Courtney/Matt (Lilycove Hideout): They still aren't changing their teams. Avoid this fight in Omega Ruby, go ahead with any good Electric STAB in Alpha Sapphire.
Gym #7 - Tate and Liza (Mosdeep City, Psychic-type): Assuming you are not underleveled and have a sufficient amount of special attack EVs, Pikachu can easily OHKO Solrock (lower special stats than Lunatone) with a Light Ball-boosted Grass Knot. Partner Pikachu up with another powerful attacker like Swampert, Sceptile, Gyarados, Azumarill, etc, and this battle will hopefully be over in one turn. A more conservative option would be to Volt Switch out and give your partner the opportunity to gang up on Solrock.
Maxie/Archie (Seafloor Cavern): Mightyena's only attacking move is Take Down, so if it Thunderbolt doesn't OHKO, then you will not be in any danger. Both Weezing and Muk, however, can easily OHKO Pikachu with Explosion and Gunk Shot, respectively; just Volt Switch out. Simply Thunderbolt Crobat into oblivion; be wary that it will most likely outspeed, but none of its attacks comes close to OHKOing Pikachu. Maxie's ace, Mega Camerupt, should naturally be avoided like the plague; however, if you are playing Alpha Sapphire, then feel free to destroy Mega Sharpedo with Thunderbolt.
Primal Groudon (Cave of Origin, Omega Ruby only): Nope. Since Primal Groudon is Ground/Fire, then Pikachu can only deal about 50% with Grass Knot and promptly get OHKO'd in return.
Primal Kyogre (Cave of Origin, Alpha Sapphire only): If your Pikachu is overleveled, has a +Special Attack nature, has a good amount of Special Attack EVs, then it can OHKO with Thunder. If one of these is not true, then just Volt Switch out.
Gym #8 - Wallace (Sotopolis City, Water-type): This matchup is very easy for Pikachu, which naturally outspeeds Wallace's whole team. Thunderbolt easily destroys Luvdisc, Sealeo, and Seaking; Grass Knot takes care of Whiscash. Milotic is a little trickier case, however: Thunderbolt has a chance of falling short of the KO, whereas the opposing Hydro Pump will probably OHKO. You can go for the Thunder if you want, but that is obviously pretty risky. Probably just Volt Switch out.
Wally (Victory Road): Icicle Crash smashes Altaria and Roselia, easily OHKO'ing both. If your Special Attack is high enough, then Thunderbolt Delcatty to death. Magneton will deal a lot of damage with Tri Attack, but you can potentially 2HKO it with Thunderbolt. It should be noted that Brick Break is a guaranteed 2HKO here even with minimal EV investment. Volt Switch out of Mega Gallade since its Special Defense is through the roof.
Elite Four Sidney (Ever Grande City, Dark-type): Every single one of Sidney's Pokémon has the potential to OHKO our electric rodent, so tread very carefully. A maximally invested Thunderbolt has a chance of OHKO'ing both Mightyena and Absol, but it by no means a guarantee. It's up to you whether you want to risk it or not. In the same way, a minimally invested Icicle Crash against Cacturne has a possibility of OHKOing. Shiftry is a tad bulkier and thus will not fall to a single Icicle Crash, so you are going to want to Volt Switch out of there. Sharpedo, of course, easily falls to Thunderbolt.
Elite Four Phoebe (Ever Grande City, Ghost-type): Dusclops will avoid the 2HKO from Thunderbolt, but cannot do anything in return. Both of the Banettes are easily outsped and OHKO'd. Be very cautious of Sableye: you have the chance of OHKOing with Thunderbolt, but if you fail, then its Foul Play will OHKO (this move DOES take into account the x2 Attack boost from Light Ball). Dusknoir is in a similar boat to its younger brother: Pikachu can easily 2HKO with Thunderbolt and Dusknoir cannot come close to KOing a full health Pikachu.
Elite Four Glacia (Ever Grande City, Ice-type): The only thing that Pikachu can reliably OHKO here is Walrein, and even then, you must have a substantial amount of Special Attack EVs in order to do so. All others are 2HKO'd with Thunderbolt, but you can tank at least one hit. Figure out which threat you want Pikachu to eliminate, and tread carefully.
Elite Four Drake (Ever Grande City, Dragon-type): Icicle Crash easily OHKO's Altaria and both Flygons. Kingdra's AI is programmed to always lead with Yawn, so you should be able to 2HKO with a combination of Thunderbolt + Volt Switch. Salamence is very risky to engage due to its Intimidate; a minimally invested Icicle Crash with probably not OHKO, whereas Salamence in return can easily OHKO Pikachu with Dragon Rush.
Champion Steven (Ever Grande City, Steel-type): For starters, two of Steven's Pokémon have Sturdy, which means that it is very beneficial for Pikachu to have Stealth Rock and/or Spikes support. Both his Skarmory and Aggron have Sturdy, but his Skarmory is easy to take out whereas the Aggron will surely OHKO you. Claydol is very bulky and will almost definitely tank a Grass Knot and then OHKO you with Earth Power. Against Cradily, you can 2HKO with Icicle Crash while avoiding the OHKO from any of its attacks, but just be wary of Icicle Crash's miss chance. Both Armaldo and Mega-Metagross can tank at least one Thunderbolt and then OHKO with their powerful offensive attacks; it is best to simply Volt Switch out unless you would like to suicide Pikachu at the very end.
Post-Game: Rayquaza can be taken on with Icicle Crash, but you really should just throw a Ball instead. It basically has a 100% catch rate. Deoxys? Nope. You will be outsped.
Cosplay Pikachu is given to you at level 20 and will know Quick Attack, Electro Ball, Thunder Wave, and whichever special move you choose: it gets Meteor Mash as Rock Star, Icicle Crash as Belle, Draining Kiss as Pop Star, Electric Terrain as Ph. D, and lastly Flying Press as Libre. The absolute best is Icicle Crash, which provides the most coverage. At level 23, Pikachu learns Double Team, which is decent but gimmicky option. At 26 is Spark, and and 34 is Discharge. 42 is the all optional Thunderbolt, which should sit on any specially oriented Pikachu permanently, whereas Wild Charge at 50 is an interesting option for physical attackers. On the contrary, Agility learned at level 45 is not that useful, as Pikachu will either outspeed and kill or be killed in return. Similarly, Light Screen is not as good on Pikachu as it would be on a fully evolved Pokémon, because its defenses will still be too low. Thunder is at 58, but you already have the TM anyways.
For TMs, Grass Knot is really good. Volt Switch is an all-purpose move that makes Pikachu a very good lead when equipped with the Light Ball. If you're pressed for Fighting coverage, but don't want to resort to Flying Press and/or want a different special move, Brick Break is the go-to option. Dig may be useful in the short term, but is later outclassed by better moves.
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.
Cosplay Pikachu Ratings
What Nature do I want? Pikachu can honestly work with most natures, though you don't want it losing any of its Speed. A lowered Defense or Special Defense is really handy instead, since Pikachu will not really need to tank hits.
How good is Cosplay Pikachu in a Nuzlocke? It can be overwhelmingly strong for an unevolved Pokémon in its base form, though it doesn't get to that point until the Light Ball comes around, and even then its defensive potential is terrible. A strategical Pokémon and a good lead, as well as fun to use, though it will be just plain good, not absolutely amazing.