Okay, I did some quick calculations, and the weakest possible Glaceon (without a special attack decreasing nature) at level 60 should be able to beat the most specially bulky Garchomp at 62, with Ice Beam in one hit, no matter the nature. This doesn’t include items, like berries.
The frailest possible Glaceon has a 43.8% chance of dying from the strongest possible Garchomp that uses either Giga Impact, Dragon Rush, or Earthquake, so you might want to bring a focus sash/band. A critical hit guarentees a KO on all of Cynthia’s Garchomp’s moves.
In terms of availability, Eevee is in Hearthome City, and both the Ice Rock and Snorunt are in route 217. (according to Serebii)
I’ve not gone over Frosslass since it’s worse in literally every way except speed, but Glaceon will be able to take a hit as long as it’s not the weakest possible Glaceon and it doesn’t get crit, so it doesn’t need the speed.
The weakest possible Froslass will always 2HKO the specially bulkiest possible Garchomp, but the frailest possible Froslass will die to the strongest possible Garchomp and the weakest possible Garchomp. The bulkiest possible frosslass with a balanced spread between physical and special defense will always die in two hits from the weakest possible garchomp. (252 hp, 170 def, 88 spdef, max ivs). Without a balanced spread it’ll die from either dragon rush (earthquake, or giga impact) or flamethrower
But go with whatever you like best! Personally, despite Froslass being the objectively worse choice, I’d go with it since I like its design more. So what design do you like best?
edit: Froslass is also level 60, and they’re underleveled because usually, at least in my playthroughs, I’m 2-4 levels under the Champion’s Pokemon.
