At first glance, you'd be forgiven for passing over Bug teams. The omnipresent Stealth Rock can be difficult for Bug-type teams to deal with, as most Bug-types are weak to it, with several (namely Mega Pinsir and Volcarona) being 4x weak to it. The Bug type also has the lowest average base stat total of all types, and Bug-type moves are resisted by seven types. Common weaknesses and a lack of good walls mean that Bug is limited to purely offensive teams; attempting to run a balanced or stall Bug team will end badly.
Despite those flaws, Bug is still an excellent type to use in Monotype. If hyper offense is your preferred playstyle, Bug is the type for you; with many powerful sweepers and wallbreakers such as Mega Pinsir, Volcarona, Buzzwole, Scizor, Mega Scizor, and Heracross, they can put immense pressure on slower, more balanced teams, such as balanced Normal. With strong priority users in Mega Pinsir and Scizor, frail offensive teams aren't safe either. While Bug-type moves are widely resisted, all of the viable Pokémon for Bug have alternate STAB types they can use. In fact, it's not uncommon to see Bug teams with only a couple of their Pokémon running Bug-type moves. Bug is also one of the few types to have no hopeless matchups. While Bug certainly has difficult matchups (more on that later), it always has a way to play around them.
Team Building
Mega Evolutions
HP65
ATK155
DEF120
SPA65
SPD90
SPE105
Mega Pinsir is the best Mega Evolution Bug has. Above-average Speed, massive Attack, and Swords Dance make it a phenomenal wallbreaker and late-game cleaner on Bug teams. Balance and stall teams can quickly be overcome by its Aerilate-boosted Frustrations, and with Aerilate-boosted Quick Attack, it's not helpless against frail offensive teams either. No glass cannon, Mega Pinsir also packs respectable base 65 / 120 / 90 defenses, so it can take neutral hits pretty well. While it does compete with Scizor and, to a lesser extent, Heracross for the Mega slot, Pinsir is often the superior choice due to its greater firepower and superior Speed tier, which lets it revenge kill threatening Pokémon such as Mega Charizard Y and Landorus, combined with the amazing neutral coverage of Frustration and Close Combat. Its Aerilate-boosted Quick Attack also lets it revenge kill Pokémon such as Keldeo and opposing Volcarona once they have taken some chip damage.
HP70
ATK150
DEF140
SPA65
SPD100
SPE75
Superb bulk in conjunction with its wonderful defensive typing makes Mega Scizor very difficult to stop late-game, while a massive base 150 Attack stat allows it to rip through almost anything that doesn't resist its attacks after a Swords Dance. While base 75 Speed is fairly average, it has access to STAB Bullet Punch, which is also boosted by Technician. Mega Scizor excels in the matchup against Normal, as it can break through the Chansey / Staraptor / Porygon2 core balanced Normal runs, while frailer, more offensive Normal teams struggle to take boosted Bullet Punches. It can also sweep Fairy teams after a Swords Dance, with only Klefki and Babiri Berry Clefable able to stomach its attacks, and the former loses if it lacks Foul Play. While it does have to compete with Pinsir, which boasts more Speed and has a superior offensive typing, for the Mega slot Mega Scizor makes up for the power drop by being much harder to revenge kill and less susceptible to Stealth Rock.
HP80
ATK185
DEF115
SPA40
SPD105
SPE75
Mega Heracross's sky-high base 185 Attack stat combined with access to Swords Dance makes it an insanely threatening wallbreaker. Its ability Skill Link transforms Pin Missile, Rock Blast, and Bullet Seed into effectively 125-Base Power attacks that also break Substitutes, Focus Sashes, Sturdy, and Multiscale. Its Fighting STAB moves let it decimate Steel teams, with only max Speed Heatran able to stop its onslaught; even Skarmory is cleanly OHKOed by a +2 Close Combat once Sturdy has been broken. Mega Heracross can shred balanced Normal teams as well, OHKOing just about everything with its Close Combat. Even defensive Staraptor is cleanly OHKOed by a +1 Rock Blast. However, it is often passed over, as Mega Pinsir boasts far better Speed, while Mega Scizor has a much better defensive typing. Both also have access to STAB priority, which Mega Heracross lacks, leaving it highly reliant on Sticky Web.
Hazard Control
HP75
ATK125
DEF100
SPA70
SPD80
SPE45
Armaldo is a staple on Bug teams. Not only can it remove the dreaded Stealth Rock with Rapid Spin, but it can set it up as well. A high base 125 Attack stat means it can still hit fairly hard, even without investment. A secondary Rock typing removes its Fire and Flying weaknesses, which lets it check Choice Scarf Victini, a hugely threatening Pokémon to Bug teams. It can also scare away potential Defoggers such as Zapdos and Staraptor with its STAB Stone Edge. Armaldo's role compression is what makes it so good on Bug teams; no other Pokémon can both check Fire- and Flying-types and provide hazard control at the same time.
HP75
ATK90
DEF140
SPA60
SPD60
SPE40
While Armaldo is usually the superior spinner, as it has far greater offensive presence and the ability to check Fire- and Flying-types, Forretress does have a few niches. For one, it isn't weak to the very Stealth Rock it aims to remove. It can also set up Toxic Spikes and has access to a slow Volt Switch to bring in a teammate safely. However, it is still extremely passive.
Utility Pokémon
HP70
ATK77
DEF60
SPA97
SPD60
SPE108
Galvantula is the best offensive Sticky Web setter Bug teams have access to. It boasts great speed and a 91% accurate STAB Thunder courtesy of its ability Compound Eyes, which forces out Defoggers such as Skarmory, Mantine, Staraptor, and Mandibuzz. While Araquanid can also set up the hazard, the aforementioned traits mean Galvantula is usually the superior choice; running Galvantula also frees Araquanid up to run its Assault Vest set. Galvantula is not deadweight outside of setting Sticky Web either; it also eases the matchup against Flying, as between Thunder and Hidden Power Ice, the only Pokémon on a Flying team able to switch into it is Zapdos. It can also 2HKO any Toxapex, which walls Mega Scizor and Volcarona that lack Psychic.
HP68
ATK70
DEF92
SPA50
SPD132
SPE42
Araquanid's niche on Bug teams comes from its ability Water Bubble. This gives Araquanid an immunity to burns and a 50% boost to its Water-type moves, letting it hit much harder than its lackluster Attack stat would have you believe. More importantly, it also weakens Fire-type moves, granting Araquanid a pseudo-Fire resistance, making it the only Pokémon on a Bug team able to switch into Mega Charizard Y. It checks many other Fire-types as well, such as Heatran, opposing Volcarona, and Blacephalon, which could otherwise do massive damage to a Bug team. It is most commonly seen running a bulky attacker set with an Assault Vest, although it can set up Sticky Web if your team requires it.
Offensive Pokémon
HP80
ATK125
DEF75
SPA40
SPD95
SPE85
HP107
ATK139
DEF139
SPA53
SPD53
SPE79
Heracross and Buzzwole serve largely the same role on Bug teams: a revenge killer that can also clean late-game thanks to their respective abilities, Moxie and Beast Boost. Their secondary Fighting typing removes their Stealth Rock weakness and lets them dish out massive damage against Rock, Normal, Steel, and Ice teams. Choosing between the two comes down to whether you prefer Heracross's superior Speed, access to Pursuit, and ability to threaten Psychic teams with its STAB Megahorn versus Buzzwole's superior physical bulk and ability to threaten Ground and Dragon teams with its access to Ice Punch.
HP85
ATK60
DEF65
SPA135
SPD105
SPE100
Good speed, huge Special Attack, and, most importantly, access to Quiver Dance make Volcarona a fearsome late-game sweeper. Its secondary Fire typing makes Volcarona an invaluable asset to Bug teams; on its own, it changes the match up against Steel from dicey to tipped in Bug's favor. Should Volcarona get a Quiver Dance up, it can make quick work of a Steel team, with only Heatran able to offer resistance. Even then, offensive Heatran can be OHKOed by a +1 Hidden Power Ground once its Air Balloon has been popped. Finding space to set up isn't difficult either—Volcarona's typing lets it set up on many common Pokémon, such as Mega Sableye, Scizor, Alolan Nintetales, Celesteela that lack Air Slash, Clefable, and Mega Venusaur. It can also serve as an excellent lure; with Psychic and Hidden Power Electric in the final two slots and a Psychium Z, Volcarona can lure in and KO Toxapex and Mantine at +1, both of which would otherwise wall it.
HP70
ATK130
DEF100
SPA55
SPD80
SPE65
Scizor's wonderful defensive typing, great bulk, and huge base 130 Attack stat make it a common threat on Bug teams. It is usually seen running a Choice Band set with Bullet Punch, Superpower, U-turn, and either Knock Off or Pursuit. This set can serve a multitude of different roles on your team; Technician-boosted Bullet Punch lets it revenge kill, a relatively slow U-turn lets it safely bring in a teammate, massive Attack lets Scizor smash through anything that doesn't resist its moves, Pursuit lets it trap Psychic- and Ghost-types, such as Latias and Gengar, and Knock Off lets it cripple Pokémon that rely on their items such as Porygon2 and Chansey. It is usually preferred over its Mega Evolution, as running base Scizor also lets you use Mega Pinsir.
Mega Pinsir can put immense pressure on a Dark team, as between Aerilate-boosted Frustrations and Close Combat, there is almost nothing that can stomach an attack from it after a Swords Dance. Even Mandibuzz is cleanly OHKOed by a +2 Frustration after Stealth Rock damage. Scizor can also wear down Dark teams with its STAB U-turn; in addition to doing heavy damage to anything not named Mega Sableye or Mandibuzz, it will keep the momentum on Bug's side. Tyranitar can be problematic, as nothing on a Bug team wants to take a Stone Edge from it, but Scizor, Heracross, and Buzzwole can all keep it in check.
Against a Grass team, victory is all but guaranteed for Bug. Nothing on a Grass team can switch into Mega Pinsir, and if it gets a Swords Dance in, it can easily sweep Grass teams by itself, only fearing Choice Scarf Rotom-C. Volcarona can decimate Grass teams by itself once it gets a Quiver Dance, which it shouldn't have much trouble doing; Ferrothorn, Mega Venusaur, and Tapu Bulu locked into a Grass- or Fighting-type move provide ample opportunities. While Cradily and Mega Venusaur take neutral damage from Fire Blast, neither can stomach a +1 Inferno Overdrive. Mega Scizor only fears Hidden Power Fire from Mega Venusaur, offensive Celebi, and Serperior—nothing else on a Grass team can OHKO it.
Ice teams have no response to Scizor; even Avalugg has a good chance of being 2HKOed by Superpower after Stealth Rock damage. Swords Dance Mega Scizor can sweep Ice teams easily, being able to OHKO everything on Ice with Superpower or its Technician-boosted Bullet Punch at +2. Volcarona can do much the same, and it can easily set up on Alolan Ninetales and Choice item Mamoswine locked into Icicle Crash, Knock Off, or Superpower. Heracross and Buzzwole can do massive damage to Ice teams with their Fighting STAB moves; only Avalugg and Froslass can avoid a 2HKO from them, and Avalugg must have full HP and Froslass needs to be wary of Stone Edge.
Pursuit Heracross is instrumental in this matchup, as it can force out Heatran and pop its Air Balloon. Additionally, not many Pokémon on Steel can avoid a 2HKO from its STAB Close Combat. Removing Heatran's Air Balloon clears the way for Volcarona to sweep; after a Quiver Dance it can OHKO everything commonly seen on a Steel team with Fire Blast or, in Heatran's case, Hidden Power Ground at +1. Finding space to set up is pretty easy, as defensive Swords Dance Mega Scizor, Ferrothorn, and Celesteela that lack Air Slash provide ample opportunity. Choice Scarf Excadrill with Rock Slide can put pressure on a Bug team, but Scizor, Mega Scizor, Heracross, and Buzzwole can easily tank it and OHKO with Superpower or Close Combat in Heracross's case.
Psychic's primary weapon against Bug is Victini, which can 2HKO everything on a Bug team. One-on-one, Choice Scarf variants can be handled by Armaldo and Araquanid, although both need full HP to properly check it. Once Victini has been removed, it should be fairly straightforward for Bug: Volcarona can decimate Psychic teams if it runs Bug Buzz, Choice Band Scizor can 2HKO everything on Psychic teams, Heracross can do massive damage with its STAB Megahorn, and Mega Scizor is nigh impossible for Psychic teams to stop once Victini has been removed.
Bad Matchups
A very tough match up, Bug is forced to rely on Armaldo and Araquanid to have a chance against Fire teams. Both of them are weak to all hazards and neither has reliable recovery, thus both are quickly worn down. Against powerful wallbreakers such as Choice Band Infernape, Choice Band Victini, Blacephalon, and Mega Charizard Y, Bug teams can only take so much punishment. Mega Charizard X can sweep Bug teams fairly easily and can easily set up on the likes of Scizor and unboosted Volcarona. Opposing Volcarona can also sweep Bug teams once Araquanid has been removed. Keeping Stealth Rock up will help you in this matchup, as almost everything commonly seen on Fire teams is weak to it. Mega Pinsir is also a great asset to you in this match up, as it can put immense pressure on Fire teams; once Torkoal has been removed, nothing on a Fire team (bar the rare Rotom-H) can stomach an attack from it, and it outruns everything except for Blacephalon and Infernape, both of which fear a Quick Attack.
Arguably Bug's hardest matchup, Flying teams have many Pokémon that cause trouble for Bug teams. Celesteela, Skarmory, and Zapdos can all stop Mega Pinsir and Mega Scizor from sweeping, while Mantine and Dragonite can stop Volcarona (the former has to be wary of Hidden Power Electric, though). Armaldo and Galvantula can help in this matchup, although Flying still has Pokémon that can handle both of them; Celesteela and Skarmory can wall Armaldo, while Zapdos can take any attack from Galvantula and OHKO it with Heat Wave after some chip damage. As with Fire, keeping Stealth Rock up will serve you well, and although Sticky Web is useless, Galvantula is very useful to dent Flying teams with its STAB Thunder and Hidden Power Ice.
Nihilego is a huge threat to Bug teams. It can 2HKO everything commonly seen on Bug teams with its STAB Power Gem, and with a Choice Scarf it can outrun Mega Pinsir and +1 Volcarona. While Scizor can dispatch it with a Bullet Punch, Scizor also lets Toxapex in for free. Your best bet is to use Pursuit Scizor to try to trap the Ultra Beast, but even then it's still a prediction game; should Nihilego stay in, Scizor will be cleanly 2HKOed by Power Gem. Mega Pinsir is helpful in this match up, as it can break through the Mega Venusaur / Alolan Muk / Toxapex core after a Swords Dance, although it still needs to watch out for Crobat, which outruns Mega Pinsir, can take a +2 Quick Attack even after Stealth Rock, and can OHKO with Brave Bird after some chip damage.
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Good Cores
Thanks GnralLao for your help with this section.
Dual sweeper core
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OR
Volcarona @ Firium Z
Ability: Flame Body
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Quiver Dance
- Fire Blast
- Giga Drain / Bug Buzz
- Hidden Power Ground
Pinsir-Mega @ Pinsirite
Ability: Moxie
Happiness: 0
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Swords Dance
- Frustration
- Quick Attack
- Close Combat
Scizor-Mega @ Scizorite
Ability: Light Metal
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature / Adamant Nature
- Swords Dance
- Bullet Punch
- Bug Bite
- Superpower
The idea is that one paves the way for the other to sweep. Volcarona can take out Celesteela, Skarmory, and Zapdos, all of which trouble Mega Pinsir and Mega Scizor, while Mega Pinsir can remove Chansey, Toxapex, Mantine, and Pelipper. Mega Scizor can take out Chansey, Tyranitar, Nihilego, and Mega Diancie for Volcarona. Which moves you run on Volcarona depends on which Pokémon you pair it up with. If you pair it up Mega Pinsir, use Bug Buzz in the third slot, Hidden Power Ground in the fourth slot, and a Firium Z. If you pair Volcarona up with Mega Scizor, use Psychic and Hidden Power Electric in the final two slots with a Psychium Z; this lets it lure in and OHKO Toxapex and 2HKO Mantine for Mega Scizor with a +1 Shattered Psyche and Hidden Power Electric, respectively.
Volcarona + Pursuit Trapper
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OR
Volcarona @ Firium Z
Ability: Flame Body
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Quiver Dance
- Fire Blast
- Giga Drain / Bug Buzz
- Hidden Power Ground
Scizor @ Choice Band
Ability: Technician
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Bullet Punch
- U-turn
- Superpower
- Pursuit
Heracross @ Choice Scarf
Ability: Moxie
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Megahorn
- Close Combat
- Pursuit
- Stone Edge
Using Pursuit is a great way to soften up opposing Pokémon so that Volcarona can sweep more easily later in the match. Heracross can force out Heatran, allowing you to hit it with Pursuit and pop its Air Balloon, thus letting Volcarona OHKO it with a +1 Hidden Power Ground. Scizor can outrun and OHKO -1 Speed Victini (hence the Jolly nature) with its Technician-boosted Pursuit, allowing Volcarona to muscle through the opposing team later in the match. If using Scizor in this core, it's highly recommended to use Armaldo on the team to switch into Victini, allowing Scizor to Pursuit trap it after the V-create stat drops.
Conclusion
Packing a plethora of powerful Pokémon, Bug is one of the best hyper offensive types you can use. While they may be but humble insects, they are most certainly not easily squashed. Underestimate Bug at your peril.
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OR 
Volcarona @ Firium Z
Pinsir-Mega @ Pinsirite
Scizor-Mega @ Scizorite
Scizor @ Choice Band
Heracross @ Choice Scarf