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It completely depends on their sets.
If Galvantua doesn't miss (91% acc), it can 2 shot Ariados with Thunder or possibly 1 shot it with Choice Specs.
Ariados has a better chance at 1 shoting Galvantua with Choice Band Poison Jab, but if it doesn't make the KO first turn it gets outsped. Might be worth running a Scope Lens with high crit moves since Sniper guarantees a 1HKO with a crit. Also, Megahorn will 1HKO with as little as Silver Powder but you have a decent chance to miss.
If Galvantula doesn't carry a Specs or Life Orb, odds are definitely in Ariados' favor since it can afford bulk investment, potentially turning Thunder into a 3HKO with max HP and an Assault Vest, while still promising a 1 or 2HKO in return.
