I've never used Galvantula until this gen and I'm loving him.
I feel like he's a good enough pick for OU, particular since he faces a lot of threat from Skill Link and Fake Out in UU. I've run him as my opening Pokémon on Showdown at OU and I've lost 4 of around 20 or so battles, with Galvantula having a significant impact to disrupting the opponents.
Galvantula @ Focus Sash
Nature: Modest
Ability: Compound Eyes
4HP / 252 SpA / 252 Spe
- Sticky Web
- Volt Switch
- Bug Buzz
- Thunder
Sticky Web will be a priority to slow down the opponent's Pokémon and if I force a switch, Galvantula suddenly becomes a little monster, able to OHKO/2HKO a ton of the most common spinners, including Tentacruel, Starmie, Donphan and even Forretress, however Donphan will pose a large problem with Earthquake/Stone Edge.
If they switch in for the spin, Galvantula will outspeed them thanks to Sticky Web and they'll either have to lose their spinner to remove my web or remove my Galvantula before spinning (which is only doable by Donphan and Starmie, the latter after Stealth Rock and only with sash/assault vest, which is HIGHLY unlikely)
Galvantula vs other openers and spinners:
Tentacruel:
252+ SpA Galvantula Thunder vs. 252 HP / 0 SpD Tentacruel: 276-326 (75.8 - 89.5%) -- guaranteed 2HKO after Black Sludge recovery
vs
0 SpA Tentacruel Scald vs. 4 HP / 0 SpD Galvantula: 109-129 (38.6 - 45.7%) -- guaranteed 3HKO
Thanks to Galvantula's superior speed, you'll only have to take one hit of Scald to remove Tentacruel from the equation - meaning the opponent has the choice of either spinner away Sticky Web or dealing some damage to Galvantula, but not both.
Forretress:
252+ SpA Galvantula Thunder vs. 252 HP / 0 SpD Forretress: 244-288 (68.9 - 81.3%) -- guaranteed 2HKO after Leftovers recovery
vs
4 SpA Forretress Volt Switch vs. 4 HP / 0 SpD Galvantula: 25-30 (8.8 - 10.6%) -- possibly the worst move ever
Don't switch Forretress onto Galvantula.
Ever.
Ferrothorn:
252+ SpA Galvantula Bug Buzz vs. 252 HP / 0 SpD Ferrothorn: 117-138 (33.2 - 39.2%) -- 100% chance to 3HKO
vs
4 Atk Ferrothorn Gyro Ball (103 BP) vs. 4 HP / 0 Def Galvantula: 80-94 (28.3 - 33.3%) -- 0% chance to 3HKO
Even the mighty Ferrothorn cannot take too many hits from Galvantula before being forced to switch.
Starmie:
252+ SpA Galvantula Thunder vs. 0 HP / 4 SpD Starmie: 368-434 (140.9 - 166.2%) -- guaranteed OHKO
vs
252 SpA Life Orb Starmie Surf vs. 4 HP / 0 SpD Galvantula: 242-285 (85.8 - 101%) -- 6.3% chance to OHKO
Unfortunately for Galvantula, Starmie WILL outspeed him in a straight battle.
Fortunately for Galvantula, if Starmie switches into Sticky Web, then Galvantula can shatter her with a 91% accuracy Thunder.
Donphan:
252+ SpA Galvantula Bug Buzz vs. 252 HP / 4 SpD Donphan: 199-235 (51.8 - 61.1%) -- 95.3% chance to 2HKO after Leftovers recovery
vs
252+ Atk Donphan Earthquake vs. 4 HP / 0 Def Galvantula: 256-303 (90.7 - 107.4%) -- 43.8% chance to OHKO
Donphan is one of few common spinners who stands a big chance against Galvantula, but if Galvantula is sashed, Donphan has to make the choice between spinning away the Sticky Web or dealing a chunk of damage to Galvantula (possibly activating the sash) before going down, in a similar scenario to Tentacruel.
Main checks/counters:
Anything that can resist both electric and/or bug, particularly ground types - Pokémon such as Hippowdon, Nidoqueen and Tyranitar - disrupt Galvantula a lot. Assault Vest Tyranitar in particular will annihilate Galvantula without taking much damage back. Carbink will survive long enough to take four thunders - assuming they all hit, meaning the little fairy pebble has more than enough time to set up safely on Galvantula.
Heatran is a large opening threat to Galvantula - easily landing a 2HKO without taking much in reply, whilst a surprise Toxic Spikes Roserade will melt Galvantula with her Sludge Bombs, however Galvantula can switch out on a number of these threats with Volt Switch without taking any damage.
Fake Out and Skill Link are large chinks in Galvantula's seemingly solid opening display, with Pokémon such as Ambipom, Cloyster, Cinccino and Mega Heracross causing big problems with their powerful Rock Blasts and others such as Mienshao and Hitmontop with their Fake Outs - all mostly UU Pokémon, however.
If Galvantula does find himself in UU, he'll find it hard to make a large impact with Steelix, Cinccino, Ambipom, Mienshao and Hitmontop all lurking around, whilst OU holds a less early problems in comparison.