Eevee is received as a gift from Bebe in Hearthome City, but it is only available before the postgame in Platinum.
Glaceon has an impressive amount of Special Attack and pretty good defensive stats. Sounds great, huh? And then you notice how late the Ice Rock is, and all the good feelings just drain away. Not to mention, those defensive stats don't make up for such poor Speed and HP (not to mention pure Ice is a terrible defensive type). My advice? It's best to evolve your Eevee into anything else.
Gym #3 - Fantina (Hearthome City, Ghost-type): Eevee will have to be ground all the way up to level 29 to learn Bite if it wants to be able to do anything here. If you're willing to do that, it can Bite its way through Duskull, even if burnt. Haunter will require Eevee to not be burnt, though, and Confuse Ray + Hypnosis means it's probably more trouble than it's worth. Mismagius is out of Eevee's league (well, without healing at least) as it could very well Psybeam spam Eevee to death. If only it didn't have that Sitrus Berry...
Rival (Hearthome City): Protip: Teach Eevee Return and max out its happiness if you want it to be able to do much here. Avoid Staravia; even with maxed Return and Adaptability there's no guarantee that Eevee will be able to take out Staravia first, and nerfing it with Growls just gives it turns to build Double Teams. Ponyta's doable with sufficiently high powered Returns, provided Eevee doesn't get burned. While Return (or Quick Attack with extremely good luck and Adaptability) will theoretically work against Roselia its Stun Spore and health restoring moves leave it very much in the hands of the RNG. Even if Eevee's best move is Quick Attack it can handle Buizel, though those with Run Away will need to be close to full health to pull it off. Battling Prinplup safely requires Eevee to either be able to outspeed it or have Adaptability, though note that having its Attack dropped at all leaves Eevee's victory very much in doubt. Avoid Monferno because it has Mach Punch. Eevee could, in theory, Return spam Grotle to death; however a single crit from Grotle will wreck Eevee's chance to win - a very likely possibility, given Razor Leaf's heightened crit chance.
Gym #4 - Maylene (Veilstone City, Fighting-type): Please don't use a fragile Normal-type in a Fighting gym, okay?
Rival (Pastoria City): Much is the same as the previous fight: Staravia and Monferno are still to be avoided and Prinplup/Grotle are a gamble, the latter moreso. Ponyta's a bit more RNG-based; one (or two if Eevee has Adaptability) Stomp-induced flinches will keep Eevee from obtaining victory. Return should be close to or at full power by now, allowing Eevee to handle Buizel wonderfully. Roselia's more manageable now that it's lost its Stun Spore.
Gym #5 - Crasher Wake (Pastoria City, Water-type): Gyarados is far too mighty for an Eevee. Technically Eevee could manage against Quagsire but it would require no Speed drops, no being Yawned and no self-inflicted confusion damage, which is very unlikely. Avoid the Floatzel; simple Aqua Jet spam has a chance of knocking out Eevee before Eevee can do the same and that's with Adaptability.
Cyrus (Celestic Town): Sneasel is too brutal with its Ice Punch;avoid. Eevee can at least attempt the Golbat, though a single crit from Golbat or self-hurt from confusion will nullify Eevee's chance for victory. Murkrow's Drill Peck tears Eevee apart; stay away.
Rival (Canalave City): Now that his team's fully evolved they're too much for an Eevee to handle. The only possible exception is Roserade, but this requires outspeeding as well as avoiding both GrassWhistle and Leech Seed.
Gym #6 - Byron (Canalave City, Steel-type): If and only if Eevee has enough of a Speed investment to outspeed Magneton it can defeat it with two Digs. Eevee can't battle Steelix or Bastiodon though; the former has Earthquake to discourage diggers, the latter carries a hard-hitting Stone Edge as well as Metal Burst and both are physically bulky as hell.
Saturn (Lake Valor): Golbat is theoretically manageable with Return, though an early use of Toxic and/or crits could screw Eevee over easily; naturally one with Adaptability has a lot more wiggle room. If Eevee has Shadow Ball (Return's stronger, but only before Iron Defense) it can just defeat Bronzor. Don't even think about trying to face down the Revenge-toting Toxicroak.
Mars (Lake Verity): Mars's Golbat is identical to Saturn's and thus runs on the same strategy. Unlike Saturn's Bronzor, Mars's is not a clear-cut victory, though still fairly likely; to win without crits/confusion hax it'd have to hit with Extrasensory every turn and not receive any Special Defense drops.Purugly Slashes poor Eevee to ribbons.
Gym #7 - Candice (Snowpoint City, Ice-type): Finally, Eevee gets to be a Glaceon! Ice Beam or Return (all things being equal their power is the same) will handle the Sneasel. Piloswine goes down in two Ice Beams and the only way it can one-shot Glaceon is with a crit Stone Edge; if you think it's worth the risk go right ahead. Glaceon will need two uses of Blizzard or two Icicle Plate/NeverMeltIce boosted Ice Beams to knock out Abomasnow before it can do the same with Focus Blast; like with Piloswine a crit is lethal, though with guaranteed hail in play Abomasnow will have trouble even hitting. Theoretically, a few Shadow Balls should off Froslass, though you'll have to contend with its irritating Double Team + Snow Cloak antics. Then again, that goes for most Pokemon you'd pitch against it, and Glaceon has its own Snow Cloak to its advantage.
Cyrus (Galactic HQ): Glaceon can gradually grind Sneasel down with Ice Beam if it has to, though if it's hit by Screech at all it should retreat for safety's sake. Crobat and Honchkrow are no problem, just fire Ice Beams until they die.
Saturn (Galactic HQ): Put Golbat on ice (Beam). Bronzor can be shot down with Shadow Balls - even if it spams Gyro Ball Glaceon can take it. Avoid Toxicroak; that Brick Break clobbers Glaceon.
Mars and Jupiter (Spear Pillar, tag battle with rival): Glaceon can handle the pair of Bronzor with Shadow Balls - aim for Mars's first, as it has Light Screen. Each Golbat will go down to a single Ice Beam, though if your rules don't allow for healing be mindful that Jupiter's has Mean Look. Back out when the aces arrive; Skuntank's carrying Flamethrower and Purugly has Thick Fat to foul your sweeping efforts.
Cyrus (Distortion World): Don't use Glaceon against Houndoom for obvious Fire-related reasons. Glaceon can tank a Heat Wave (provided it doesn't crit) and OHKO Honchkrow with Ice Beam. Crobat, likewise, falls to a single Ice Beam. Avoid Gyarados because if it spams Waterfall (unlikely, but best to be safe), Glaceon is going down. Also avoid Weavile as while Glaceon struggles to damage it quickly Weavile has no such qualms.
Giratina (Distortion World): Glaceon would need to use Substitute in order to avoid death via Shadow Force spam and even then there's a good chance of having its setup ruined by any of Giratina's other moves. Best to tag it once with Ice Beam and switch, or just let someone else handle it altogether.
Gym #8 - Volkner (Sunyshore City, Electric-type): Barring paralysis hax, Glaceon can shut down Jolteon with Ice Beam. Avoid Raichu as you both 2HKO the other with Ice Beam and Focus Blast respectively; it's unlikely it'll land both Focus Blasts, but best to be safe, y'know? As Luxray has Rivalry, Glaceon will need to be female or faster to avoid being killed by Fire Fang; if it is, go right ahead with Ice Beam. Also avoid Electivire as it'll punch out Glaceon's lights with Fire Punch before it can do the same with Ice Beam.
Rival (Pokémon League): Glaceon can take a Close Combat from Staraptor and respond with a OHKOing Ice Beam. Don't have Glaceon battle Floatzel as its Brick Break will have Glaceon down first. Snorlax is too bulky a beast for the likes of Glaceon. Heracross is off-limits; one Close Combat will kill Glaceon. Roserade is an easy (and soon to be icy) target, assuming you don't get Grasswhistled. Rapidash and Infernape cannot be fought by Glaceon for flamingly obvious reasons. Neither can Glaceon battle Empoleon simply due to lack of a means to kill it quickly enough. At least Torterra goes down to an Ice Beam before it can even act.
Elite Four Aaron (Pokémon League, Bug-type): Yanmega goes down to one Ice Beam. Avoid Scizor; Iron Head hits hard and you don't really have much to hit it with anyway. Also avoid Heracross as its Close Combat is a clear OHKO. Vespiquen shouldn't bee too much trouble; you have about a 50/50 chance at one-shotting with Ice Beam and can take a few Power Gems. Two Ice Beams should take out the Drapion, though beware of crits.
Elite Four Bertha (Pokémon League, Ground-type): Ice Beam spam may work for Whiscash, though if you fail to two-shot it, retreat. (BTW, holding a NeverMeltIce/Icicle Plate ensures a 2HKO.) One Ice Beam will kill Gliscor twice over. Hippowdon's another approximately 50% chance of one-shotting with Ice Beam, though if it survives a crit Stone Edge will one-shot Glaceon, so be careful. Ice Beam is a solid 2HKO on Golem during Sandstorm and a OHKO otherwise while the best it can do is a 3HKO, so no problems there. Only battle Rhyperior if Sandstorm isn't active, otherwise it'll survive an Ice Beam and wreck with Rock Wrecker.
Elite Four Flint (Pokémon League, Fire-type): DON'T, YOU FOOL!
Elite Four Lucian (Pokémon League, Psychic-type): Mr. Mime's probably gonna set up a LIght Screen at some point, though if you luck out and it doesn't, Shadow Balls polish it off. Espeon's gonna be hitting too hard with Psychic for Glaceon to handle. Bronzong's too damn bulky even before it starts setting up Calm Minds. Alakazam's a no-go; it takes two Shadow Balls to down it and two Psychics or Focus Blasts will finish Glaceon first. Neither can Glaceon battle Gallade, not with it carrying both Drain Punch and Stone Edge.
Champion Cynthia (Pokémon League): Two Ice Beams will shut down that Spiritomb. If you fail to one-shot Roserade with Ice Beam, switch; it's only a small chance that it'll score a 2HKO with Sludge Bomb, but it's still there. As tempting as Togekiss is, don't; Aura Spheres kill you first. Lucario is the same story as Togekiss. Milotic has Mirror Coat to discourage Special Attacks and there's no way Glaceon can beat it physically. Glaceon can survive a non-crit hit from any of Garchomp's attacks (and even a crit has a fair chance of being survivable) and responds with an Ice Beam for the win, so if nothing else you gotta love its ace-killing capabilities, right?
Post-Game: Heatran is a massive NOPE and as for the Frontier? Ehhhh, Glaceon's a bit limited for such challenges.
When you receive Eevee it'll have Tackle, Helping Hand, Sand-Attack and Growl, none of which are that good. At level 22 it learns Quick Attack which at least is an improvement over Tackle. At level 29 it learns Bite allowing it to fight ghosts if necessary. At level 36 it learns Baton Pass though all it can pass is Substitute and Double Team. Take Down, Last Resort and Trump Card at levels 43, 50 and 57 respectively are not worth holding Eevee back from evolution even further.
Glaceon learns Quick Attack and Bite at the same levels but instead learns Ice Shard at level 36 which can be a handy finisher if needed. At level 43 it learns Ice Fang and while the STAB is nice it runs off the wrong stat. Last Resort comes at level 50 again. Mirror Coat is at level 57 though it's pretty risky to be using in a Nuzlocke. Hail, Blizzard and Barrier would all be nice on Glaceon, but with them coming at levels 64, 71 and 78 they're simply too late for the main game.
For TMs, Eevee will appreciate Return for power and Dig for Rock and Steel types, though there are better Pokemon for both. Ice Beam is a staple for Glaceon and Shadow Ball provides coverage. Hail is necessary if you want to take advantage of its ability (not that you should rely on it). If using Hail, Blizzard makes for a wonderful combo. Substitute can be combined with Baton Pass.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
What Nature do I want? If you plan to have it battle as an Eevee, a Speed decreasing nature is best. If using it solely as Glaceon, Attack decreasing natures are good also.
Which ability do I want? Eevee definitely wants Adaptability; even if the boost is minor, it still helps more than Run Away. Not that it'll matter once it evolves, as Glaceon gets Snow Cloak regardless.
At what point in the game should I be evolved? If you plan on running Substitute + Baton Pass you'll have to wait until Level 36 for the latter, though if you've been raising Eevee this whole time it'll be close to that level anyway by the time you hit Route 217. Just don't delay it any longer than that, for the love of logic!
How good is Glaceon in a Nuzlocke? Given how late it has to wait to become combat viable and how other Ice types become available at the same time, I'd say not very good.