Joltik is available at Chargestone Cave as the most common Pokémon, with an encounter rate of 39% on the first floor and 36% on the second.
Joltik has to be the cutest Bug-type of the series, but in addition to that, this tiny spider is blessed with a great unique typing that works really well in a region full of Fighting and Dark Pokémon. With access to the wonderful ability that is Compoundeyes, the move Thunder and high Speed, Joltik and Galvantula can become the line of the most fearsome bugs in all of Unova.
N (Chargestone Cave): This is a really bad start for Joltik. Boldore OHKOs Joltik without problems, Ferroseed resists its attacks and can revenge kill it, Joltik is a mirror match that isn't going to do anything except bore you and Klink can be beaten with some Electrowebs, however it can hurt your spider too.
Gym #6 - Skyla (Mistralton City, Flying-type): For this fight you have to follow two simple steps. Step one, evolve your Joltik to Galvantula. Step two, OHKO everything in her team with Electro Ball. A Joltik probably could take Swanna with Volt Switch, but the chance of being OHKOed due to the spider's fragility are high.
Cheren (Route 7): Volt Switch/Electro Ball for Unfezant, Dewott and Simipour, Signal Beam for Simisage, Liepard and Servine. Don't try battling Simisear and Pignite as they have STAB Fire moves to OHKO your Galvantula, although Set mode players can use Volt Switch on Pignite and Simisear to change Pokémon while dealing damage. Be careful about Speed-lowering natures and bad Speed EVs and IVs, because Simisear's speed is quite close to Galvantula's.
Gym #7 - Brycen (Icirrus City, Ice-type): If you have a Compoundeyes Galvantula, buy the Thunder TM and teach it to the spider. Vanillish is a joke and Galvantula is going to destroy it with Thunder before it can do anything, Volt Switch for Cryogonal if you are playing Set as it has a really high Special Defense to withstand most of your attacks (change when the game would normally let you in Switch mode). Beartic can be a problem as it has base 110 Attack and enough bulk to survive a Thunder, so use Volt Switch to escape or don't let Galvantula enter that fight.
Bianca (Route 8): Stoutland has bulk and strength, so use Volt Switch to escape from it. Samurott and Simipour are OHKOed with Thunderbolt or Thunder, while Serperior, Simisage and Musharna are OHKOed and 2HKOed with Signal Beam. Like with Cheren, Set Mode players can use Volt Switch to change Pokémon while battling Simisear and Emboar.
Gym #8 - Drayden/Iris (Opelucid City, Dragon-type): All of their Pokémon resist Galvantula's best moves and aren't OHKOed by Signal Beam, not to mention that they have high Attack and dangerous moves that are going to kill your spider in one or two hits. Galvantula should not enter this fight unless you have a free switch in and you've already weakened the Pokémon. Even then, avoid using Galvantula in this fight, unless you have no other options.
Cheren (Route 10): Same as before, Electric moves for Unfezant, Samurott and Simipour, Signal Beam for Serperior, Simisage and Liepard and Volt Switch to escape from Simisear and Emboar. Easy. Again, be careful about whether you actually outspeed the Simisear.
Elite Four Shauntal (Pokémon League, Ghost-type): You can use Thunder and Thunderbolt for Cofagrigus to 2HKO it at best, but the best chance for the spider is Shauntal's Jellicent, one decent Electric-type move is going to be enough to OHKO it. Don't waste your time (and your Galvantula's safety) with Chandelure and Golurk, they destroy Galvantula.
Elite Four Grimsley (Pokémon League, Dark-type): Thunder and Thunderbolt are going to do a lot of damage to Scrafty and Bisharp, while Liepard is going to go down with one Signal Beam, but remember that the cat knows Fake Out, so use that to buff your Special Attack or heal. Krookodile can be OHKOed with boosted Special Attack and 2HKOed with unboosted Signal Beam, although the crocodile is a really risky threat as it going to do a lot of damage to Galvantula with Earthquake, with a high chance of knocking it out.
Elite Four Marshal (Pokémon League, Fighting-type): High base Attack with powerful Rock moves. Let your Galvantula take a rest and see how a Psychic-type enjoys this battle.
Elite Four Caitlin (Pokémon League, bedroom, Psychic-type): Signal Beam for Reuniclus, Musharna and Gothitelle, use Thunderbolt or Thunder for Sigilyph. GG.
Reshiram/Zekrom (N's Castle): Reshiram is a no and while Zekrom isn't going to use any super-effective moves over Galvantula, is going to hit really hard and a critical hit is probably going to finish off your spider. Use another Pokémon for these two or throw a Master Ball, as you will need to capture them eventually, in order to progress with the plot.
N (N's Castle): Same as the above for N's Reshiram/Zekrom. Vanillish is knocked down with Thunder/Thunderbolt. Volt Switch for Carracosta to break its Sturdy and change to a Pokémon that can resist Rock and Water. Go with Volt Switch with the Klinklangs as one of them is a Zoroark with Flamethrower, if the Zoroark is out, beat the real gear with your STAB Electric move. For Archeops, everything depends in how quick your Galvantula is, as any Electric move is going to OHKO the bird, but any move the bird has is going to OHKO the spider.
Ghetsis (N's Castle): Volt Switch to escape from the Cofagrigus to let a Steel-type bring down its Toxic-stall strategy... and probably this is going to be Galvantula's last move as most of the Pokémon Ghetsis has a good few super-effective moves (Eelektross has Flamethrower, Hydreigon has Fire Blast and Bisharp has Stone Edge) or hit hard enough to put the spider in trouble. Galvantula can probably battle the Seismitoad with Signal Beam or if you teach Galvantula Hidden Power, wish for a Grass- or Fighting-type to OHKO either Seismitoad or Bisharp.
Post-Game: The rematches at the League work in a similar way. Other than that, Galvantula with Volt Switch is still a good battle lead.
A freshly captured Joltik can come with Fury Cutter, Electroweb, Bug Bite, Gastro Acid or Slash as its moves. Electroweb is a mediocre starting STAB move for the time you caught Joltik, but its secondary effect - lowering the enemy's Speed - makes it worthwhile until you have better alternatives. Bug Bite doesn't take a lot of advantage from Joltik's low Attack stat, but can work with the Audino grind as this kind of Pokémon usually carry Berries. At level 29 Joltik learns its first good move, Electro Ball; this move's damage varies depending on the Speed difference between the user and the enemy - knowing that Galvantula is one of the fastest Pokémon in Unova, this move is going to do a lot of damage. Signal Beam appears at level 34 and is the best special-oriented Bug-type move Joltik can get in the normal game. Level 40 has Agility and level 44 has Sucker Punch, which are really disappointing as Galvantula is going to outspeed most of its rivals all the time and it doesn't have enough Attack to make a good use of Sucker Punch as Dark-type coverage. Sucker Punch can be useful in the short term though, if an opponent unexpectedly outspeeds and doesn't kill your Galvantula. Level 54 comes with Discharge which is a little bit late as players at that time are going to have access to Thunderbolt or Thunder, but not a bad move thanks to its 30% chance of paralyzing the rival. For players that play the post-game battles, at level 60 you get Bug Buzz, which is going to be their upgrade for Signal Beam and the last move this spider can learn.
Via the Move Reminder of Mistralton City, Joltik can recover Thunder Wave, a paralyzing move that works with Electro Ball as it greatly lowers the opponent's Speed.
In the TM department, Joltik and Galvantula don't have a lot to look at like most Electric-types in the series. Thunder is the best move a Compoundeyes Galvantula can take and Thunderbolt is the best choice for Unnerve spiders. Volt Switch is a really good move for Set mode players as it makes good use Galvantula's high Speed, letting the spider run out of the battle without suffering damage and dealing a good amount to its enemy. The last STAB option would be Charge Beam as it can increase the spider's Special Attack while dealing damage. Volt Switch is typically preferred though. Double Team, Swagger and Attract can work as supportive moves to hax the battle at your favour, but really risky ones as Joltik and Galvantula aren't really bulky to survive a lot of hits. Hidden Power works as the coverage moves for the spider, knowing that Energy Ball TM is in the post-game, Fighting- or Ice-type are some of the best Hidden Powers for this Pokémon as they let Galvantula battle Steel or Ground and Dragon-types that resist its normal STABs. Rain Dance and Toxic are other options if you finished with a bad Hidden Power and don't want to put three Electric-type moves in your Pokémon.
Recommended moveset: Thunder [Compoundeyes] / Thunderbolt [Unnerve], Signal Beam, Hidden Power / Rain Dance / Toxic / Thunder Wave, Volt Switch / Charge Beam
Water-types: They are probably one of the best partners for Joltik as they hit Fire and Rock for supper effective and can still deal damage to other dangerous Pokémon that the spider can't tackle like Ground or Steel-types. At the same time, Joltik and deal with Grass-types and can damage other Electric-types with Signal Beam.
Ground-types: Another really good partners for Joltik, as with Water, they hit for super effective both Fire and Rock, resisting the second one. In addition, Ground deals with Steel types and Electric types that could put a Joltik under a lot of pressure. Again, Ground's weakness to Grass and Water is covered by the little spider and helps them to hit Flying types too.
Bulky Pokémon: Galvantula is frail, like really frail, so pairing it with Pokémon that can get hits easily helps Joltik to switch easily from one teammate to another with Volt Switch. Joltik will always prefer Pokémon that can resist both Fire and Rock-type moves so it can have a big brother to hide behind. Another preference will be for bulky Pokémon that use physical moves more than special ones.
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.
Joltik Line Ratings
What Nature do I want? Modest or Timid. The first one is going to rise Galvantula's decent Special Attack to let it OHKO more things and the second one is going to allow it to outspeed all of Unova and escape anything with Volt Switch. Other options that lower the Attack stat to get some bulk like Bold (+Def) and Calm (+SpDef) are welcome too. Speed-lowering natures are still usable, but not recommended because of how frail the line is.
Which Ability do I want? Compoundeyes, having a 91% accuracy Thunder is one of the best things you could ask in an Electric-type Pokémon and this give you a better chance to hit rivals that raise the evasion. Unnerve is good too, but there aren't many trainers that put Berries on their Pokémon, so isn't as useful as Compoundeyes.
At what point in the game should I be evolved? Before Skyla. She is really weak to the Electric-type and a Galvantula with Electro Ball or Volt Switch is able to sweep her Gym in less than a minute, so evolve your Joltik as soon as possible for her. If not, before the battle with Cheren at the entrance of Twist Mountain you should have evolved your spider to Galvantula. There are no moves that you need to wait for.
How good is the Joltik line in a Nuzlocke? In a region where you can find a Dark-type in one out of two battles, a Bug-type is a really good option and a Bug-type combined with one of the rarest and hardest to capture type in Unova is awesome. Joltik probably lacks a good coverage movepool, but can have access to Compoundeyes Thunder, its great Speed to hit and run with Volt Switch and a decent base Special Attack of 97 makes the Joltik line a really good Pokémon that can live without that coverage. A frail, but fast and powerful spider that doesn't let its enemies to hit it before they finish up fried by Galvantula's moves, one of the best glass cannons for Set mode players.