Grubbin can be found on Routes 1, 4, 5 and 6 in both games. Charjabug can be found on Blush Mountain.
The perfect mix: an early game bug with the latest possible evolution, rivalling only the likes of Volcarona. Despite going for most of the game as Charjabug, this line is fairly capable of taking hits on both the physical and the special side; on the other hand, its moves will rarely hit for much damage, and the necessity to have the Eviolite on at all times before the Vast Poni Canyon prevents Charjabug from using Z-moves in the vast majority of matchups. This handicap is further aggravated by the discouraging rendition of the long-awaited fully evolved Vikavolt in the immediate matchups after it reaches its last stage: its typing is far from ideal against the last Totem Pokémon, and its Speed puts it at a disadvantage against Lusamine. With the Elite Four being the only redeeming factor of an otherwise not bad, but rather lackluster Pokémon, Vikavolt is stuck in the "just average" tier for almost the entirety of the game.
Hau battle #2 (Iki Town): Vice Grip has the best damage output against Pichu, scoring a 2-3HKO on average, against Pichu's relatively weak Thunder Shock. Grubbin also has a decent matchup against Rowlet, dealing slightly higher damage than its Tackle, though it will need healing or to be switched out at some point during the battle; it loses against Popplio and Litten instead, particularly the latter, whose Ember is far stronger than any of Grubbin's moves.
Ilima (Hau'oli City, Normal-type): Grubbin's Bite can 3-4HKO Yungoos before Yungoos deals enough damage with Tackle, or a combination of Tackle and Leer, though low damage rolls combined with enemy critical hits can endanger Grubbin. The matchup against Smeargle is feasible if it does not know Ember; in that case, its Tackle will be about as powerful as Grubbin's Bite after the Technician boost, so Grubbin will be able to deal some damage before healing or switching out. Ember, however, puts Grubbin at a decisive disadvantage.
Totem Gumshoos/Raticate (Verdant Cavern): Grubbin is very disadvantaged against Totem Gumshoos, taking much more damage than it can deliver. Totem Raticate is also impossible, unless Grubbin already knows Bug Bite; in that case, the move will 2-3HKO, but the matchup is not risk-free as the ally Rattata may either Leer or add up damage with Hustle-boosted Quick Attack.
Hau (Route 3): Grubbin's Bug Bite wins against Pikachu and Popplio, 2-3HKOing the former and 3-4HKOing the latter on average, but loses against Rowlet and Litten, which can hit for super effective STAB with Peck and Ember respectively.
Hala (Iki Town, Fighting-type): Vice Grip has better damage output than Mankey's Pursuit, and can 3-4HKO; however, Mankey also knows Focus Energy, which can make the fight more even. The matchup against Makuhita is similar: even a five-hit Arm Thrust is just barely stronger than Vice Grip, but Sand Attack can get in the way of Grubbin actually hitting. Grubbin's power also falls short against Crabrawler: despite the Fighting resistance, All-Out Pummeling still inflicts great damage and even regular Power-Up Punch contributes to complicating the matchup; already having Spark or using Breakneck Blitz can only help so much, since Grubbin would still be at a non-negligible disadvantage against Hala's ace.
Ilima (Hau'oli City, Normal-type, optional): Grubbin's Bug Bite is stronger than Smeargle's Ember, scoring a 2HKO, while Smeargle may only 3HKO and Tackle is already in the 3-4HKO range. However, Grubbin should come nowhere close to Gumshoos, which can even one-shot with Breakneck Blitz.
Hau (Paniola Town): Grubbin's Bug Bite still 2-3HKOs Pikachu without a problem, but it loses against all three of Hau's possible starters. While Acrobatics may net a 2HKO against Dartrix, so can Dartrix with Peck, and Torracat delivers super effective STAB damage via Fire Fang. Even Brionne is considerably stronger than Grubbin, despite having gained access to Spark: Breakneck Blitz can chip away almost half of Grubbin's health, and Aqua Jet finishes the job two turns later. Not even paralysis achieved through Spark can subvert the matchup, since Aqua Jet is a priority move.
Gladion (Route 5): Grubbin loses against Zubat, since both Wing Attack and Spark are 2HKOs, but Zubat is faster. Type: Null is also far beyond Grubbin's reach in sheer power.
Totem Wishiwashi (Brooklet Hill): Charjabug has trouble against Totem Wishiwashi despite its Electric weakness. While Spark remains a 3HKO even counting the Totem's Defense increase, Water Gun can even 2HKO under the rain, and comes close to an OHKO with an ally's Helping Hand. In addition, if Totem Wishiwashi's health is below half after the first blow, it will call Alomomola instead of Wishiwashi, which can spam Heal Pulse. Charjabug may attempt to hit and paralyse the Totem, but is highly unlikely to accomplish anything beyond its first turn of battle.
Battle Royal (Royal Park): An unevolved Grubbin thoroughly loses against all three opponents in the Battle Royal. An already evolved Charjabug has better chances, but still takes up to about 45% in damage from Rockruff's Rock Throw; if it must participate in the battle, it should aim to KO Rockruff as soon as possible. Do not have Charjabug fight if Hau's starter is Torracat, the combination of Rock Throw and Fire Fang is deadly.
Totem Salazzle (Wela Volcano Park): There is little that Charjabug can do against Totem Salazzle's Flame Burst; although it can only 3HKO, and so can Charjabug's Spark, the Totem is faster and does not fight alone. Charjabug may try to shoot a Spark and paralyse it, but is unlikely to be able to stay in the second turn without risking it, unless the Totem uses Torment first.
Totem Lurantis (Lush Jungle): Charjabug can 2HKO Totem Lurantis with Acrobatics, if it is not holding an item. Since the first hit will leave the Totem below half health, it will call Castform instead of Trumbeak; this plays in Charjabug's favour, since Trumbeak knows Rock Blast and would easily win. After beating the Totem, if Castform put up Sunny Day, Charjabug must switch out: Castform also knows Weather Ball. Backup is required regardless, since a combination of Synthesis and Sunny Day on the second turn will also force the switch and leave the Totem alive.
Plumeria (Akala Outskirts): An Eviolite Charjabug can easily two-shot Golbat with Spark. It is unlikely that it will also beat Salandit, though, unless healed; Flame Burst is nearly a 2HKO, and the damage taken from Golbat will do Charjabug in before long.
Olivia (Ruins of Life, Rock-type): Do not. All of Olivia's Pokémon significantly outdamage Charjabug, even if it holds the Eviolite.
Lana (Konikoni City, Water-type, optional): Chinchou can only be 2-3HKOed with Bug Bite, since its ability is Volt Absorb. Shellder is easily 2HKOed with Spark; Araquanid can nearly kill with Hydro Vortex, though, so it should be handled by a different Pokémon.
Kiawe (Royal Avenue, Fire-type, optional, Sun only): If supported by the Eviolite, Charjabug can 2HKO Fletchinder with Spark. Its matchup is too disadvantageous against Growlithe and Marowak, though: Growlithe has Intimidate and knows Fire Fang, while Marowak's Flame Wheel outdamages even Crunch.
Mallow (Lush Jungle, Grass-type, optional, Moon only): It is better for Charjabug to not hold items in this battle; the Eviolite is not necessary, since it resists most of the moves Mallow's Pokémon know, and Acrobatics is a clean 2HKO against all three of Mallow's Pokémon, closing in on an OHKO against Phantump. Steenee may Teeter Dance before that and have Charjabug hit itself, so a backup or healing items should still be available.
Nihilego (Aether Paradise): Venoshock deals about as much damage as Spark, being on average a 4HKO. Charjabug is unlikely to win against Nihilego unless healed almost every other turn, but it can hold off for the five turns needed for Nihilego to flee.
Hau (Malie City): Charjabug's Bug Bite is a clean 2HKO against Raichu, with a small OHKO chance, but the Eviolite is needed to survive two Psychics from it, which is only possible if neither of them is a critical hit. Bug Bite also beats Leafeon in three to four hits, while Spark 2-3HKOs Vaporeon; of all the Eeveelutions, only Flareon is forbidden territory, due to Fire Fang. Torracat also roasts Charjabug, but Brionne can be 2HKOed with Spark and Dartrix is 3-4HKOed by Acrobatics, despite the decrease in power from holding an item.
Totem Vikavolt (Hokulani Observatory): Fighting Totem Vikavolt with Charjabug is a bad idea. Despite the possible Eviolite use, the Totem's aura more than compensates for the defensive boost, leading even Charjabug's X-Scissor outdamaged by Bug Bite.
Guzma (Malie Garden): An Eviolite Charjabug can do just enough against Golisopod: by leading with Protect, it can avoid the likely First Impression, and Spark is a 3HKO against Golisopod's Razor Shell 4HKO. Although one Swords Dance takes Golisopod to 2HKO range, its Emergency Exit will negate the boost on the second Spark. Charjabug can also simply start with Spark and continue until Golisopod drops. if strong enough to withstand the damage (which depends on Charjabug's stats). Acrobatics 3HKOs Ariados even with an item being held, though Charjabug should not hit it if its health is below one quarter, as it knows Sucker Punch.
Totem Mimikyu (Thrifty Megamart): Charjabug should not battle against Totem Mimikyu. Its aura makes it much more powerful, and none of Charjabug's moves can deal significant damage to it before Charjabug itself gets destroyed, either by Play Rough or by the damage dealt by Mimikyu's allies.
Plumeria (Route 15): Charjabug 2HKOs Golbat with Spark, but needs to avoid Salazzle due to Flame Burst.
Guzma (Shady House): Charjabug can no longer take on Golisopod: the newfound EV investment in Attack on Guzma's end makes Razor Shell more powerful than Spark; even though Wild Charge scores a 2-3HKO, Golisopod may still win by using Swords Dance or scoring a critical hit, a combination of circumstances that is difficult to prevent, especially with Razor Shell having a higher-than-average critical hit rate. Gigavolt Havoc running off Spark or Wild Charge is still not an OHKO, although it is guaranteed to activate Emergency Exit. Ariados is fairly weak and can be 3HKOed with Acrobatics. The Eviolite is needed against Ariados, thereby making the fight against Golisopod impossible, but does not compromise the Acrobatics 3HKO.
Gladion (Aether House): An Eviolite Charjabug fears little from Golbat's Acrobatics, and can 2HKO it with Spark; Sneasel is most likely OHKOed by X-Scissor, or 2HKOed if not. X-Scissor and Wild Charge are both 4HKOs against Type: Null, the latter coming close to 3HKO range, though Charjabug may not be able to finish the fight, depending on how many Defense drops it suffers from Crush Claw: it can handle one, but two or more flip the matchup upside down. Charjabug cannot use any Z-moves against Type: Null, since forgoing the Eviolite means losing the matchup.
Nanu (Malie City, Dark-type): Charjabug can 3HKO Sableye with Spark or Wild Charge, but suffers a 3-4HKO from Power Gem even with the Eviolite, so the matchup is not ideal. It can, instead, OHKO Krokorok with X-Scissor, though the move will only 2HKO if Charjabug is under the effects of Krokorok's Intimidate. Alternatively, Charjabug may use Savage Spin-Out running off X-Scissor to OHKO Krokorok even if Intimidate is in effect; Charjabug can survive Earthquake from Krokorok, even a critical one in most cases. The matchup against Persian is difficult, due to Fur Coat reducing X-Scissor to 2-3HKO range, and Intimidate further lowering Charjabug's offensive capability (if not negated); Black Hole Eclipse can also take away over half of Charjabug's health, so Charjabug is only recommended for use against Persian with the Eviolite attached and as a switch-in after the Z-move has already been used, especially since Power Gem is also a 3HKO bordering on a 2HKO.
Faba (Aether Paradise, tag battle with Hau): Charjabug should hold the Eviolite and start by 1-2HKOing Ledian with Spark, or OHKOing with Wild Charge; although Ledian itself is not dangerous, its Reflect and Light Screen combination make Faba's Pokémon much worse. Even with the Eviolite attached, Charjabug is likely to struggle against Slowbro, whose Psychic is a 3-4HKO against Charjabug's X-Scissor 3HKO; Wild Charge should not be used against it, as its high HP will cause Charjabug to suffer considerable recoil damage. Instead, Wild Charge is required against Bruxish, which can 2HKO with Psychic Fangs; however, Charjabug's health needs to be high before dealing the blow, in order to be able to take both Psychic Fangs and the Wild Charge recoil. Otherwise, Charjabug can OHKO Bruxish with Savage Spin-Out, though this implies it will not be holding the Eviolite, and that makes it even less viable against the Ledian and Slowbro pair. Without using the Z-move, Wild Charge is likely to OHKO Bruxish, but this is not guaranteed. Hypno is 2HKOed once again by X-Scissor.
Guzma (Aether Paradise): It is not recommended even for an Eviolite Charjabug to go against Golisopod; Wild Charge can 2-3HKO, but the 3HKO is much more likely and Razor Shell inflicts massive damage, scoring an OHKO with a critical hit after Swords Dance. Gigavolt Havoc running off Spark or Wild Charge will send Golisopod into Emergency Exit range, but critically lowers Charjabug's usability against the rest of Guzma's Pokémon. Ariados can be 3HKOed with Acrobatics even if Charjabug is holding the Eviolite; Masquerain is 1-2HKOed by Wild Charge, though another move should be chosen to finish it off if Wild Charge does not do so in one turn. If Charjabug is holding the Electrium Z, it can OHKO Masquerain, but needs to be at 80% of its health or above to take a Bug Buzz beforehand. Pinsir cannot be beaten, it has good Defense and its X-Scissor is too strong.
Lusamine (Aether Paradise): Charjabug is disadvantaged against Clefable: Moonblast outpowers all of its moves even with the Eviolite attached, Charjabug is slower, and Metronome may pull strong super effective moves if used. Instead, it can 2HKO Lilligant with X-Scissor; not Mismagius, however, as its Power Gem 2HKOs Charjabug before it can do the same with Crunch. Mismagius can be beaten only with Black Hole Eclipse powered by Crunch, but this puts Charjabug with lower-than-average Special Defense at a serious risk of being OHKOed by Power Gem, due to the lack of Eviolite. Milotic's Hydro Pump is also too strong; a high damage roll, paired with Wild Charge recoil, could even off Charjabug and using Spark is not recommended, since paralysis would activate Milotic's Marvel Scale. Bewear must be left alone, its power and ability ridicule Charjabug.
Hapu (Vast Poni Canyon, Ground-type): Dugtrio must be left alone, as it would 3HKO Charjabug with Earthquake; so does Flygon with Earth Power. Gastrodon has full EV investment in HP and Defense, and would be horribly painful for Charjabug to take down. Mudsdale can nearly kill with Tectonic Rage, and takes next to no damage from Charjabug's moves, as its ability is Stamina. Overall, Charjabug cannot do anything in this battle even if it has the Eviolite attached. Charjabug can, however, be evolved before the battle with a Rare Candy; a Vikavolt that knows Energy Ball can effortlessly 2HKO Dugtrio and Mudsdale, 3HKO Flygon and even OHKO Gastrodon, though it will likely need to be healed once or twice in the process, as it will not outspeed anything but Gastrodon.
Totem Kommo-o (Vast Poni Canyon): Despite the long awaited evolution, there is little use for Vikavolt in this matchup: Acrobatics barely scratches 4HKO range, and the Totem's Clanging Scales is nearly an OHKO.
Lusamine (Ultra Space): All of Vikavolt's moves are severely outdamaged by Clefable's Moonblast which, due to Clefable's full Special Defense EV investment and its +Special Defense aura, inflicts even more damage than Corkscrew Crash. Vikavolt loses even against Lilligant's Petal Dance, a 2HKO, unless it OHKOs it with Savage Spin-Out powered by X-Scissor. Likewise, Mismagius can only be beaten by a Black Hole Eclipse, as all other moves fail to OHKO and its Power Gem is a 2HKO. Even Milotic 2HKOs with Hydro Pump, and must be defeated with Gigavolt Havoc, since all other moves will give it the upper hand. A Vikavolt with Speed above 80 can 2HKO Bewear with Thunderbolt if still fully healthy, but must otherwise resort to Gigavolt Havoc or Bewear itself will 2HKO it with Take Down.
Solgaleo (Altar of the Sunne, Sun only): A combination of Gigavolt Havoc and a regular Thunderbolt may KO Solgaleo, though Solgaleo's own Zen Headbutt is also a very likely 2HKO. If Solgaleo opens with Zen Headbutt, continuing the battle will be risky. Cosmic Power also complicates the matchup, reducing even Gigavolt Havoc to below 50% damage.
Lunala (Altar of the Moone, Moon only): Black Hole Eclipse running off Crunch would not be an OHKO even if Lunala's ability were negated. Vikavolt cannot win this matchup.
Gladion (Mount Lanakila): Vikavolt has a good matchup against Crobat, Weavile and Lucario; Thunderbolt is an OHKO against the first and a 2HKO against the other two. The matchup against Silvally depends on its Memory: Fire-type Silvally beats Vikavolt easily with Multi-Attack, Water-type Silvally is 1-2HKOed by Thunderbolt, and Grass-type Silvally is 2-3HKOed by X-Scissor. Optionally, Vikavolt may OHKO Water-type Silvally with Gigavolt Havoc or 2HKO Grass-type Silvally with Savage Spin-Out and X-Scissor.
Hau (Mount Lanakila): Bug Buzz OHKOs Raichu and Thunderbolt is a 2-3HKO against Komala; it can also be 2HKOed with Gigavolt Havoc and Thunderbolt combined, or the same combination with Vikavolt's Bug STAB instead. Vaporeon is either OHKOed by Gigavolt Havoc or 2HKOed by Thunderbolt, while Leafeon is simply OHKOed by Bug Buzz; Flareon must be avoided, due to Flare Blitz. Vikavolt can win against Decidueye only by combining Savage Spin-Out and Bug Buzz, else Decidueye would 3HKO with Spirit Shackle first; Incineroar and Primarina are impossible to beat, they both outspeed and OHKO Vikavolt with Inferno Overdrive (or even regular Flare Blitz) or Hydro Vortex.
Elite Four Hala (Pokémon League, Fighting-type): Vikavolt should preferably have Air Slash and not hold an item in this matchup, or switch in after Hariyama is down in order to hold an item. Hariyama's Knock Off can be outmatched only by Air Slash, which 2-3HKOs; before knocking out Hariyama, at least one Iron Defense usage is recommended, as it helps Vikavolt take hits better from Hala's other Pokémon. A Vikavolt backed by +2 in Defense easily beats Primeape, 1-2HKOing with Air Slash, as well as Bewear, 2HKOed. Poliwrath can be 1-2HKOed with Thunderbolt, though the OHKO is unlikely; Crabominable is 2HKOed by both Thunderbolt and Air Slash, though Vikavolt needs the Defense boost to survive multiple Ice Hammers.
Elite Four Olivia (Pokémon League, Rock-type): Relicanth can be OHKOed with Energy Ball or Thunderbolt, but Vikavolt loses against everything else on Olivia's team, due to the crippling Rock weakness and Olivia's Pokémon being largely built with specially defensive EV spreads.
Elite Four Acerola (Pokémon League, Ghost-type): Sableye is 1-2HKOed by Thunderbolt, while Drifblim is always 2HKOed unless it uses Amnesia. If a faster Pokémon can shoot down Drifblim, it should be used, since it can otherwise boost multiple stats and Baton Pass them to a teammate, even with Focus Energy; alternatively, Vikavolt can use Gigavolt Havoc, which will OHKO without boosts or nearly OHKO after one Amnesia, allowing Thunderbolt to finish the job the following turn. Dhelmise is 2-3HKOed by Bug Buzz. Flash Cannon has a low chance to OHKO Froslass, but if that fails, Vikavolt is unlikely to be able to take two Blizzards in a row; Energy Ball 2HKOs Palossand, and Vikavolt is likely to outspeed, but there is also a small chance that Never-Ending Nightmare may kill Vikavolt from full health, a risk to be considered.
Elite Four Kahili (Pokémon League, Flying-type): Skarmory's strongest move against Vikavolt is Slash, which is fairly weak even with its EV investment in Attack. Vikavolt can use Skarmory to set up Agility and/or Iron Defense at will; in both cases, reaching +6 is recommended, since Vikavolt's Speed is low and having a good Defense helps against Toucannon. The issue for non-Agility Vikavolt is the lack of proper coverage against Oricorio, which easily kills with Revelation Dance; a Vikavolt that can outspeed, instead, OHKOs it with Thunderbolt. Crobat is not an issue for either set, being OHKOed by Thunderbolt; Iron Defense users get past Mandibuzz's Punishment easily, while Agility users need their health to be above half to survive a Punishment before they can 2HKO with Thunderbolt, else they can OHKO it with Gigavolt Havoc. Toucannon is also OHKOed by Thunderbolt alone, so Agility sets beat it effortlessly, and Iron Defense sets that got to the +6 marker will still survive even a five-hit Rock Blast if their health is at 40% or above.
Professor Kukui (Pokémon League): Lycanroc's Stone Edge is a full-health OHKO against Vikavolt, which should abstain from leading; Ninetales can be OHKOed easily with Flash Cannon. Braviary is 1-2HKOed by regular Thunderbolt, or OHKOed with the Expert Belt attached; Vikavolt's health needs to be two thirds or higher to take Brave Bird beforehand, though. Magnezone can be 2-3HKOed with Dig; Vikavolt must use physical moves to beat it, as it knows Mirror Coat. Since Vikavolt resists all of Magneton's moves, it can set up Iron Defense and/or Agility for as long as its health levels last, though it should not push its luck, as Magnezone's Flash Cannon can lower Vikavolt's Special Defense. One Iron Defense is enough to significantly improve the matchup against Snorlax, now 4HKOable with Thunderbolt; two usages of the move are best, though, since its Crunch may otherwise compensate the boosts by lowering Vikavolt's Defense. Vikavolt must not fight Incineroar due to Flare Blitz and the respective Z-move, and also avoid Primarina, which OHKOs with Hydro Vortex; it can, however, attempt to 2HKO Decidueye with Bug Buzz, though it will need to heal or switch out if the move fails to do so, as Decidueye can otherwise 2HKO with Spirit Shackle. Alternatively, Vikavolt can secure the 2HKO by starting with Savage Spin-Out, which also has a small OHKO chance, or by being at +2 in Defense or higher, since the entirety of Decidueye's moveset is physical.
Tapu Koko (Ruins of Conflict): Tapu Koko's moves are fairly weak against Vikavolt, which can easily 3HKO with Energy Ball. If Vikavolt can use Bloom Doom, Tapu Koko can be 2HKOed instead.
Post-Game: Vikavolt has largely positive matchups against the Ultra Beasts, thanks to its resistances, and can also be fairly helpful in the League rematches.
Grubbin starts with just Vice Grip, then learns String Shot at level 4 and Mud Slap at level 7. Its initial moveset is good, but nothing special; it gets further coverage at level 10, however, through Bite. STAB rolls out easily afterwards: at level 13, Grubbin gets Bug Bite, and at level 16 it learns Spark, which receives the 50% boost only four levels later. Grubbin also learns Acrobatics at level 19, though it is likely to get access to the move via TM before it does so by level. The first move after the evolution into Charjabug is the fairly useful Crunch, at level 25, a good upgrade of Bite; although Bug Bite makes part of the Dark coverage redundant, it nevertheless does its job against Ghost-types and anything that may resist Bug moves. Further upgrades come at level 31, when Charjabug learns X-Scissor. Then Dig, at level 37, can be considered, depending on the team's needs for coverage; it can be kept over Crunch or left to be. Charjabug learns Discharge at level 43 as well, though this move is unlikely to do much; Vikavolt gets the more powerful Thunderbolt upon evolution, and Charjabug's Special Attack is not good, so Spark will deal more damage until the evolution occurs. At level 49, Vikavolt learns Agility whereas Charjabug learns Iron Defense; both moves can be useful, and the area that makes Vikavolt's final evolution possible comes around not long before said level, so deciding to postpone the evolution to get both moves is an option. The special STAB, Bug Buzz, only comes at Mount Lanakila, through the Move Reminder; another Reminder-exclusive noteworthy option is Air Slash.
As most interesting TMs are Vikavolt exclusives, Charjabug will not get much more until its evolution happens. Special moves such as Charge Beam, Volt Switch and Thunderbolt can be learned by TM, but Charjabug's Special Attack and Speed are too low to support them. Acrobatics can be taught a little earlier, and potentially also X-Scissor, but these TMs only add flexibility to Charjabug's moveset, as they would also be learned normally by level. Light Screen is a defensive option to consider, especially since Charjabug's Defense is good, despite its low base HP. Once evolved, Vikavolt obtains further coverage in the form of Energy Ball, or optionally Solar Beam if running in a team with Drought or Sunny Day users; Flash Cannon is also a viable move in a team that needs further coverage against Fairy-, Ice- or Rock-types, though Vikavolt should never fight Rock-types carelessly, and preferably never at all.
Grubbin may find some use for Breakneck Blitz in the early game, but due to the base power of Bug Bite and Spark (once evolved), the Z-move will be outclassed quickly. The next accessible crystal gives it access to Gigavolt Havoc and, not long after, Savage Spin-Out. Crunch can be turned into Black Hole Eclipse, as well, but with Charjabug's Bug STAB, Dark coverage will rarely turn out useful. Once evolved into Vikavolt, Energy Ball can be turned into Bloom Doom and Flash Cannon can be powered up into Corkscrew Crash to counter Ground-types, Fairy-types and other likely Vikavolt-reserved jobs. In the endgame, Supersonic Skystrike running off Air Slash may also have some use.
Recommended moveset: Agility / Iron Defense, Thunderbolt, Bug Buzz, Energy Ball / Flash Cannon / Volt Switch
Water-types: Water-types are common in Alola, and function as a perfect complement to Vikavolt's typing. Physical Water-types with good Special Defense are even better, as they cover for the exact stats that are not bad, but just average on Vikavolt.
Ground-types: While rarer than Water-types, Ground-types function just as well as Vikavolt's helpers, especially as physical hitters. Since they are non-redundant with Water-types, they can be run alongside them on the same team as Vikavolt.
Fast hitters: Vikavolt will almost never outspeed anything, even if EV trained in Speed, and its EVs are better spent on other stats in any case. It is better to just have a dedicated glass cannon for this job, even better if Attack is its strong suit, since Vikavolt is a special hitter.
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.
Grubbin Line Ratings
What Nature do I want? Quiet is the best option. While Modest is great in the long run, Charjabug has a very average Attack stat which, if nerfed further, will amount to very little damage for most of the game. Likewise, natures that detract from either of its defensive stats are bad, especially before the final evolution is reached; Special Attack-lowering natures are paradoxically not terrible, since Vikavolt only makes use of its Special Attack in the endgame and the base stat is so high that a 10% cut hardly matters.
At what point in the game should I be evolved? Grubbin should be turned into a Charjabug before the battle against the Totem Wishiwashi. It will then evolve again only after Hapu, potentially waiting until at least level 49 to learn Iron Defense.
How good is the Grubbin line in a Nuzlocke? Just average for most of the game, due to its extremely late final evolution. Charjabug is not bad, but its Speed and lack of offensive presence are a big issue; in addition, even after the evolution into Vikavolt, the special movepool is not great. Charjabug will somewhat help in some matchups, and Vikavolt will work fairly well at the League, but there are likely better Pokémon to choose from.