Glaceon is one of the dominant forces of hail in RU, with a Blizzard so enormously powerful that it 2HKOes everything that doesn't resist it—and that's assuming there are no entry hazards. It's surprisingly bulky, too—short of hitting it super effectively, not many attackers are able to OHKO Glaceon before it wipes them out. Glaceon can even use hail to stall with a SubProtect set, one of the most annoying sets that hail teams have to offer.
However, Glaceon certainly isn't without its flaws. Its base 65 Speed is quite low, and it has no way of boosting it. Its Ice typing is a double-edged sword, giving it nasty weaknesses that compromise its otherwise great bulk. Its movepool is very small, making it rather predictable once you know which set it's running. Finally, it requires Snover as a teammate to provide the hail that makes it viable. If you can get around these flaws, though, Glaceon will rarely disappoint you.
Given that Glaceon has the highest Special Attack of any Pokemon legal in RU play, it is no surprise that it can run a viable Choice Specs set. Blizzard is the key to this set; with STAB, Choice Specs, and hail backing it up, it becomes a perfectly accurate powerhouse of a move. Even many Pokemon that resist the move need be careful; for example, Crawdaunt is always OHKOed by Blizzard even without any entry hazards after factoring in hail damage. Being generally bulky isn't enough to save Glaceon's hapless victims either, as even max HP Spiritomb is effortlessly OHKOed, again with only hail damage for support. Whenever your opponents lack a healthy team member that has both special bulk and an Ice resistance, simply select Blizzard; it is almost certain they'll be losing a Pokemon.
Shadow Ball is used entirely for Slowking, but is still the most viable coverage move in Glaceon's shallow movepool. Ground is the preferred Hidden Power type, beating Pokemon that can take a Blizzard after Stealth Rock and hail damage such as Emboar, Lanturn, and Kabutops. Hidden Power Fire is a viable alternative, as it is Glaceon's best weapon against Steel-types that aren't weak to Ground as well as opposing Ice-types, most notably Escavalier, Cryogonal, and Rotom-F, all of which can beat Glaceon otherwise. However, Glaceon with Hidden Power Fire has no means of beating Lanturn. The final move is mostly a filler, but each option does have a few niche uses. Frost Breath is used for beating defensive boosters, such as Cosmic Power Sigilyph and Calm Mind Clefable, even after they've set up; it also gives Glaceon a backup STAB in case Blizzard runs out of PP. Baton Pass seems strange on a Choice set, but can be used if you suspect your opponent will switch so that you keep up your momentum.Team Options & Additional Comments >>>
While Glaceon is not nearly as powerful as it would be when holding Choice Specs, a STAB Blizzard from this set is still nothing that should be underestimated, as it even 2HKOes Entei after Stealth Rock damage. Choice Scarf Glaceon is as fast as Choice Scarf Emboar and physically bulkier and more powerful than Rotom-F, while also possessing a useful ability in Ice Body allowing it to survive for a longer period of time. Hidden Power Ground is the most useful Hidden Power in general for Glaceon, hitting Lanturn, Kabutops, and Fire-types, while Shadow Ball hits Slowking super effectively and has good neutral coverage on Pokemon such as Poliwrath. Shadow Ball is especially useful for not becoming locked into Blizzard if the opponent has an Ice-type resist such as Slowking, and conserving Blizzard's PP. A STAB Blizzard is actually stronger versus Pokemon such as Poliwrath, though. The last moveslot depends on preference. Frost Breath is great for beating Pokemon with Special Defense boosts that may be a threat to hail teams such as Uxie, Spiritomb, and Sigilyph. Baton Pass can be used as a pseudo U-turn to scout the opponent, though Glaceon will usually be firing off powerful Blizzards during its time on the field. With a Choice Scarf, Glaceon becomes a very effective late-game sweeper.Team Options & Additional Comments >>>
While the Choice Specs set aims to outright destroy the opposition, this set takes a more subtle and prolonged approach. The threat of Glaceon's enormously powerful Blizzards forces a lot of switches, which this set uses to set up a Substitute and, from there, takes advantage of the combined recovery from Leftovers and Ice Body, which needs only one turn after using Substitute to fully pay its cost. Because this turn is very easily bought with Protect, Glaceon can comfortably stall for 32 turns straight while the opponent can do very little about it, and with Toxic Spikes and other entry hazard support, Glaceon can easily stall an entire team to death.
While Walrein can do the same thing with more bulk and a generally better typing, Glaceon can use its powerful Special Attack stat and Blizzard to take the set in a more offensive direction. Though not as powerful without Choice Specs, Glaceon still has plenty of power, enough to easily beat common users of Taunt that would wreck Walrein. Taunt users such as Drapion and Qwilfish are easily 2HKOed by Blizzard despite the latter's Ice resistance, not to mention that other common grounded Poison-types, such as Roselia and Amoonguss, are effortlessly OHKOed. In the fourth slot, Hidden Power Ground deals with Lanturn and Steel-types very well, while Toxic prevents Cryogonal and Rotom-F from walling this set.Team Options & Additional Comments >>>
Given Glaceon's shallow movepool, it doesn't have too many other options. Wish and Heal Bell are interesting team support options, but SubProtect has four-moveslot syndrome as it is, whereas Choice Specs would need to switch out after using these moves. Fake Tears can see some use on an Icicle Plate variant of the Choice Specs set to force switches, beat defensive teams easier, and make up for the lack of Choice Specs's power, but this requires a lot of prediction to use well. Roar is an option for the SubProtect set, but this is generally better done by Walrein.
It should be noted that because Snow Cloak is banned, Glaceon cannot use any of its moves that are illegal with Ice Body. This isn't really a big loss, as the only even remotely viable move lost in this way is Water Pulse, and Snow Cloak is inferior to Ice Body even on the Choice Specs set, as it prevents bluffing the possibility of the SubProtect set. Also of note is that a super effective Water Pulse would be weaker than a neutral Blizzard; for similar reasons, Signal Beam is not recommended despite its legality.
Checks and Counters
In general, the best way to deal with the Choice Specs set is revenge killing it; unless you have one of the few Pokemon that can switch into it safely, it's easier to simply take advantage of its low Speed to at least force it out. Do note, though, that Glaceon can be surprisingly bulky, and rarely has qualms about taking advantage of a hit that isn't strong enough to KO the would-be revenge killer. Dealing with the SubProtect set can be trickier; phazing it or Encoring it are your best bets. While Taunt can work with some prediction, Glaceon is fairly likely to just use Blizzard the turn you use the move. If you have a weather move other than Hail, using it can swiftly neuter Glaceon regardless of the set and usually much of its team as well, though make sure to KO Snover first.
If you want something that can beat Glaceon regardless of the set, there are a few viable options. Specially defensive Hariyama with Thick Fat isn't 2HKOed by Choice Specs Blizzard and can either hit Glaceon super effectively or phaze it away with Whirlwind. Poliwrath and Regice also avoid the 2HKO and, with Rest, have no fear of Toxic Spikes or Toxic itself from the SubProtect set. Escavalier can also work well if it has Overcoat, though note that it needs significant investment in Special Defense to not be 2HKOed by Choice Specs Blizzard and it also must be careful of Hidden Power Fire. Rotom-F also fears Hidden Power Fire, but takes little damage from Glaceon's other moves and can scout with Volt Switch. Cryogonal can take a Blizzard and Recover off the damage, though it can do little to threaten Glaceon in return and will usually lose to Choice Specs Glaceon with Hidden Power Fire or SubProtect Glaceon with Toxic. Slowking can comfortably take a Blizzard, but needs to watch out for Choice Specs Shadow Ball and Toxic. Lanturn can use Heal Bell to rid itself from Toxic, but needs to watch for Hidden Power Ground. If all else fails, Munchlax is a nearly perfect counter, but considering how little utility it has elsewhere, don't use it unless you're really desperate for an answer to Glaceon.