Eevee is received as a gift from Bebe in Hearthome City, but it is only available before the postgame in Platinum. It can be evolved into Leafeon right away, by going back to Eterna Forest and levelling it up near the Moss Rock.
The misfortune of Grass-types is infamous in many games, and Sinnoh games are no exception. Platinum in particular, with its plethora of threats for Grass Pokémon, is not merciful at all towards Leafeon, especially over the many other available Eeveelutions; in terms of sheer type usefulness, Leafeon is possibly only better than Glaceon, and only because the latter is available at a much later point in the game, and it would be difficult to keep Eevee alive as an Eevee until that point. Type-wise, it loses even to the otherwise unfortunate Flareon, whose pure Fire typing is actually handy in a number of situations in Sinnoh. Still, Leafeon does have selling points as well: for a Grass-type, it is surprisingly strong and durable; it can be pitched against Pokémon bombs without fear, especially since it has super effective moves to hit them, and it gets access to a wider movepool than most Grass-types do, including handy Bug coverage for the otherwise probably dangerous Psychic-types. Naturally, it is not the optimal choice for players who want to make their life easier in an already difficult region to cross, but it is actually worth considering for those who want even more of a challenge.
Gym #3 - Fantina (Hearthome City, Ghost-type): A gym that makes both Eevee and Leafeon very sad. The Pokémon are not too powerful, with the exception of Mismagius, and they mostly rely on physical moves, meaning that Leafeon could theoretically do great here; in practice, however, it gets no access to any move that is not Normal-typed until level 29, when it learns Bite. If Leafeon is already evolved and knows Bite, it will 2HKO Duskull and Mismagius and also 1-2HKO Haunter, performing even better than Eevee despite Eevee's immunity to Ghost moves, as Mismagius' Shadow Ball is only a 3HKO.
Rival (Hearthome City): An unevolved Eevee will not accomplish much in this battle; it can win against Buizel and Ponyta by spamming Tackle, but Staravia has Intimidate, Roselia easily regains health with Mega Drain and the starters are all too strong. As a Leafeon that already knows Bite, instead, the matchup becomes better: while fighting against Staravia is still a poor decision, as it will cut Leafeon's Attack and it also knows Wing Attack to hit for super effective damage, Roselia and Buizel are 2HKOed easily by Bite and it can also 3HKO Ponyta, whose Ember is only a 4HKO. Grotle and Prinplup take longer to defeat, but Leafeon's Bite is still stronger than their moves. Monferno's Flame Wheel, instead, makes the matchup unwinnable; avoid it.
Gym #4 - Maylene (Veilstone City, Fighting-type): Leafeon's lacking coverage hinders it in this matchup, but the Return TM - available as a Game Corner prize - makes it better. Meditite is 2HKOed by it, and Machoke is 3HKOed. Neither of them deals much damage to Leafeon. Lucario sadly outspeeds and outdamages with Drain Punch, which also lets it recover HP, but there is a workaround: if Leafeon has been taught Swords Dance by TM as well as Return, it can boost its stats against Meditite or Machoke. Meditite is preferable, as Machoke has both Leer and Focus Energy, which may complicate the gym matchup for Leafeon. At +6 Attack, Leafeon's Return (if at full base power) can OHKO even Lucario, despite its resistance to Normal moves; Lucario's Fighting STAB, instead, achieves a 4HKO at best.
Rival (Pastoria City): Although Leafeon can technically defeat Staravia with three Returns, its Intimidate still makes it a less than preferable option, as Leafeon's matchup against the rival's other Pokémon is much better without the Attack reduction. Buizel and Prinplup both get destroyed by the newly acquired Razor Leaf, Roselia and Ponyta are 2HKOed by Return and Grotle is 3HKOed. Monferno is also 2-3HKOed depending on Leafeon's Attack, but its Flame Wheel only 3HKOs at any rate.
Gym #5 - Crasher Wake (Pastoria City, Water-type): Leafeon is one of the very few Pokémon that does not fear Gyarados. In fact, Gyarados' moves deal such little damage to Leafeon, that it can Swords Dance to its heart's content against Wake's lead. Three Swords Dances will get Leafeon to +5, considering Gyarados' Intimidate, and at that point everything is OHKOed by Return. An even faster sweep is given by one Swords Dance, which allows to 2HKO Gyarados with Return and OHKO Quagsire with Razor Leaf; in this case, however, the OHKO against Floatzel is not guaranteed, and Floatzel may outspeed. It only 3HKOs with Ice Fang, but using Gyarados to get off as many sweeps as possible is the preferred option.
Cyrus (Celestic Town): Leafeon can net a 2HKO against Sneasel, but its Ice Punch oscillates between 2-3HKO and Leafeon will likely be outsped, making the matchup risky. Golbat and Leafeon both 3HKO each other, though Leafeon is guaranteed to outspeed and can win if its HP is still full when the fight begins; same for Murkrow, with reciprocal 2HKOs instead.
Rival (Canalave City): Leafeon is not a suitable opponent for Staraptor or Infernape. It can, however, take on Heracross by using Swords Dance once and then 2HKOing with Return, since Heracross' Aerial Ace is only a 3HKO and it lacks Bug STAB; it accomplishes the same against Rapidash, as its only Fire move is Fire Spin, which Leafeon can take fairly well - even better if it already defeated Heracross, as it will not need another Swords Dance. Rapidash is, however, faster than Leafeon, so it could outspeed; do not fight Rapidash in low health. Razor Leaf 2HKOs Floatzel without problems, or OHKOs if a prior Swords Dance has been used. Empoleon's Aerial Ace is a 4HKO on average, as is Leafeon's Razor Leaf in its natural state; it can, however, turn into a 2-3HKO (a likely 2HKO, but not certain) after a Swords Dance, steering the matchup in Leafeon's favour. Roserade is another good Swords Dance setup target, as its only offensive move is Giga Drain and it is 2HKOed by Return, whereas Torterra is 3HKOed by X-Scissor.
Gym #6 - Byron (Canalave City, Steel-type): Leafeon's most used moves accomplish little against Byron, though it can have a much better matchup against him by learning Dig. Magneton is OHKOed by it, whereas Steelix is averagely only 4HKOed and does know Ice Fang; Ice Fang is also a 4HKO, but Earthquake can damage Leafeon when underground. Leafeon can work around this by setting up Swords Dances and healing in the meantime, until it hits +6 and can OHKO Steelix with Dig. At least one Swords Dance is also essential against Bastiodon, to secure the Dig OHKO; otherwise, its Metal Burst will kill Leafeon.
Saturn (Lake Valor): Leafeon can 3HKO Golbat with Return, if Air Cutter scores no critical hits. If Bronzor is sent out before Toxicroak, Leafeon should Swords Dance twice before hitting it with X-Scissor; this will grant it an OHKO against Toxicroak too, which will otherwise 2HKO with its Poison Jab. A Leafeon with Dig can 2HKO Toxicroak before it does so, regardless of Swords Dance.
Mars (Lake Verity): Same logic: Golbat can be 3HKOed by Return and Bronzor is harmless enough to offer Leafeon a Swords Dance setup opportunity. Swords Dance is not strictly necessary to win this matchup, but it will help OHKO Purugly from +4 onwards, and can also get Leafeon rid of Bronzong more quickly.
Gym #7 - Candice (Snowpoint City, Ice-type): Since Piloswine's best move, Avalanche, only 2HKOs Leafeon when Piloswine is damaged, Leafeon can use Swords Dance on the first turn. This will guarantee an OHKO with Razor Leaf, and also an X-Scissor OHKO against Sneasel and Abomasnow; in the event that Sneasel outspeeds, Leafeon can still take an Ice Punch if its health is above half. However, Froslass' Blizzard is a straight up OHKO against Leafeon, so it should not fight against Candice's ace, not even with its Attack maxed out: Froslass is fast and will most likely outspeed Leafeon.
Cyrus (Galactic HQ): Leafeon can 1-2HKO Sneasel with X-Scissor and take an Ice Punch from it, if need be. It should not attempt to set up against it, because even one Swords Dance will not suffice to get rid of Crobat, which 2HKOs with Air Cutter. Honchkrow is slower, but Leafeon would need at least two setup turns to defeat it, and its Drill Peck 2HKOs; there is not enough wiggle room for Leafeon to do a Swords Dance sweep here.
Saturn (Galactic HQ): Again, Golbat can be 3HKOed with Return, assuming no critical hits. Bronzor can be exploited for Swords Dance setups, healing confusion if needed, because confusion recoil does take Attack boosts into account. If Leafeon successfully achieves a +4 in Attack, it can one-shot Toxicroak too, or do so with Dig at +2.
Mars and Jupiter (Spear Pillar, tag battle with rival): Leafeon's participation in this battle requires some risks, but it can actually pull off a sweep under the right conditions. The Bronzor's best move is Extrasensory, a 5-6HKO against Leafeon, or 3-turn KO assuming the worst case scenario (both of them target Leafeon at the same time every single turn). If Leafeon is kept healed, or lucky enough to not always be hit by the enemy moves, it can get its Attack up to +6 so long as the Bronzor remain on stage. From there, it needs to have its confusion status healed if present, as Mars' Bronzor also knows Confuse Ray. At +6, without Reflect up, Leafeon OHKOs both Bronzor with X-Scissor, but should actually focus on beating all of Mars' Pokémon (to the left) before targeting any of Jupiter's (to the right). In fact, having Jupiter's Bronzor still on the battlefield is rather crucial for the sweeping plans to succeed. Leafeon can OHKO Golbat with Return even at +4, but having both Golbat on the battlefield at the same time leaves it vulnerable to their super effective moves; Jupiter's Golbat in particular has Sludge Bomb, a 2HKO that can kill a Leafeon below about 80% of its health. Purugly's Aerial Ace is much less problematic in comparison, scoring only a 4-5HKO; Purugly is also OHKOed by Return starting at +4; it is faster than Leafeon, so it will need to survive a hit before bringing down Purugly. If Purugly goes down before Jupiter's Bronzor and Leafeon is at +6 Attack, the game is won: it can then proceed to annihilate Jupiter's Pokémon without mercy.
Cyrus (Distortion World): Houndoom, Crobat, Honchkrow and Weavile all destroy Leafeon before it can take them down. The matchup against Gyarados is better; despite its Intimidate, it can at most 3HKO with Ice Fang or Giga Impact, so Leafeon can Swords Dance and be healed as necessary until ready to take it down. At +1 Attack, the default after suffering Gyarados' Intimidate, Return has good chances of 2HKOing; this is the best compromise, as a certain OHKO would require +5 or above in Attack.
Giratina (Distortion World): Even after several Swords Dances, Leafeon deals comparatively little damage to Giratina and is also left open to its Ominous Wind which, while only a 3HKO, can cause all of Giratina's stats to get a boost. Do not use Leafeon against it.
Gym #8 - Volkner (Sunyshore City, Electric-type): One Swords Dance usage against Jolteon will do Leafeon no harm, and allows it to 2HKO everything on Volkner's team with Dig. If Leafeon does not know Dig, it can still OHKO everything with Return, but will need two Swords Dances instead; this is also not an issue, as Jolteon is incapable of 2HKOing Leafeon. Raichu's Signal Beam is a 2HKO, though, so Leafeon must either be certain to outspeed it - which requires a Speed value of 123 or more - or above half health by the time Raichu comes, so as to be able to withstand a hit. Leafeon with natures that are detrimental to Speed should use an X Speed as precaution, as they could potentially also be outsped by Electivire.
Rival (Pokémon League): Staraptor is still off limits for Leafeon. Heracross can be beaten if it uses Close Combat, as Return will 2HKO either after a Swords Dance or after a Close Combat defensive drop; if both happen, Return becomes an OHKO, even better. Snorlax 3-4HKOs with Body Slam and knows no moves to boost its stats; Leafeon can use it to set up Swords Dances: at +6, Snorlax will be OHKOed by Return, whereas from +2 onwards the 2HKO is netted. Return still nets a 2HKO - OHKO after a Swords Dance - against Roserade, who can 3HKO at best with Poison Jab; Torterra also offers it more Swords Dance setup room, as its Crunch is not that strong and at +2 or more, X-Scissor 2HKOs it. Floatzel's Ice Fang is only a 4HKO, giving yet another Swords Dance opportunity; Razor Leaf OHKOs from +2 and up. KOing Empoleon is more complicated, but a Razor Leaf 2HKO is fairly likely starting at +2, as well, and its Aerial Ace is averagely a 4-5HKO. Leafeon should steer clear from Rapidash and Infernape, both of which can kill with their respective Fire STABs.
Elite Four Aaron (Pokémon League, Bug-type): Even with the level advantage, Leafeon struggles to tank hits from Bug-type Pokémon and cannot 2HKO any of them even with its strongest moves, except Drapion and only if it knows Dig. Leafeon should not fight against Aaron at all.
Elite Four Bertha (Pokémon League, Ground-type): Whiscash deals absolutely pitiable damage, and Leafeon can get all the way to +6 with Swords Dance in its face. After that, Razor Leaf OHKOs everything except Gliscor, which is however OHKOed by Return. Leafeon may benefit from holding the Wide Lens, to eliminate the possibility of Razor Leaf missing.
Elite Four Flint (Pokémon League, Fire-type): A Leafeon with Dig manages an OHKO against Houndoom and, if it can outspeed thanks to the level advantage, also a 2HKO against Rapidash. However, fighting against Rapidash is a poor choice, as it can use its spare turn to put up Sunny Day and subsequently turn its Flare Blitz 2HKO into an OHKO. Leafeon can do the same with Infernape, but its combination of Earthquake (when Leafeon is underground) and Flare Blitz will also off Leafeon. Flareon and Magmortar are unkillable in one hit, and will defeat Leafeon with their respective STABs. Overall, Leafeon is much better off staying away from Flint's team.
Elite Four Lucian (Pokémon League, Psychic-type): Mr. Mime can only 3HKO with Psychic and, after a Swords Dance, Leafeon can OHKO Espeon for certain. Alakazam is also OHKOed regardless of possible Attack boosts. Both Espeon and Alakazam are likely to outspeed, however, and can 2HKO with Psychic; if one of them has landed a hit, it would be wiser for Leafeon to not fight the other. Leafeon can 3HKO Gallade with any of its moves that have at least the same base power as X-Scissor, and Gallade struggles to 4HKO it; again, it will serve as a chance to set up, in case Leafeon has switched out previously or not yet participated in the battle. It is dangerous for Leafeon to fight Bronzong without a prior setup: only at +6 does X-Scissor become an OHKO, and Bronzong's Psychic is a 3HKO, which turns into a 2HKO with just one Calm Mind boost; Bronzong can be a threat for entire teams if fought by a Pokémon that cannot handle it swiftly. If Leafeon is backed by one Swords Dance, it can still 2HKO Bronzong, though.
Champion Cynthia (Pokémon League): Spiritomb's Silver Wind is a 3HKO close to 2HKO range, and Leafeon is highly unlikely to 3HKO; it should avoid Cynthia's lead. Roserade's Sludge Bomb and Togekiss' Air Slash also kill Leafeon from full health, and none of Leafeon's moves can do the same to them unless Leafeon already has one (for Roserade) or three (for Togekiss) Swords Dances supporting it; in that case, the OHKO with Return is assured. A healthy Leafeon can beat Milotic by either 2HKOing with Razor Leaf or OHKOing after a Swords Dance; both options require it to take an Ice Beam, which Leafeon can do at full health. Lucario can be 2HKOed only with Dig, or OHKOed with a Swords Dance backing up Leafeon; otherwise, it will two-shot Leafeon with Aura Sphere, and it can also chip away the remainder of its health - if already low - with ExtremeSpeed. Fortunately, one Aura Sphere is not enough to bring Leafeon to ExtremeSpeed KO range. Garchomp 2HKOs Leafeon with Flamethrower regardless of what it uses, and Leafeon can only have a not so high OHKO chance with Return with its Attack maxed out; fighting Garchomp is just not worth it.
Post-Game: Leafeon finally gets Seed Bomb, and... that is about it. The previous threats just got worse, so proceed with caution if the challenge is meant to continue into the postgame.
Eevee's movepool when received consists of Tackle, Helping Hand, Sand-Attack and Growl. Its only STAB suffers from Gen IV's base power of 35, and is not really good for anything, but it will have to do for a little while. At level 22, it learns Quick Attack as both Eevee and Leafeon; the first difference comes at level 29, when it gets Bite as Eevee, and Synthesis as Leafeon. Unless Leafeon is pressed for an early evolution, waiting until level 29 is a good idea to get better coverage for the Leafeon-to-be, as it will be unable to get any sort of physical STAB until Pastoria City. At level 36, it gets Baton Pass as Eevee or Magical Leaf as Leafeon; unless Leafeon plan on teaching Swords Dance and profit of Baton Pass shenanigans, waiting for the move is not really needed, even if its alternative is not much better, due to Leafeon's poor Special Attack. If Leafeon did wait for level 36 to evolve, then wait no longer: there is no point keeping Eevee an Eevee any more, if the plans are to evolve it into Leafeon, as it learns no other moves worthy of mention from that level onwards. Leafeon then gets Giga Drain at 43, which is about as "meh" as Magical Leaf; Leafeon will be relying on Razor Leaf most of the time, which can be relearned in Pastoria City, but since its movepool is not very wide it could actually benefit from keeping either Magical Leaf or Giga Drain for some time, just to have a better damage output against the physically bulky Rock- and Ground-types. Last Resort comes at 50, but even Return is a better option than that, and GrassWhistle, at 57, is no better due to the chronic lack of accuracy. Sunny Day comes at 64, if Leafeon gets that far, but it is not worth grinding for it; the move can be taught much more easily via TM. The last level-up move is at level 71 and it is Leaf Blade, which is an excellent move to have for Leafeon, the best possible; sadly, a lot of overgrinding is needed for Leafeon to learn it, considering the Champion's ace is nine levels lower than that. Swords Dance, at 78, is once again not worth waiting for: Leafeon will be way too overlevelled at that point, and the Swords Dance TM is available in the Game Corner long before then, at any rate.
Be sure to stop by the Move Reminder as soon as Pastoria City is reached, as it will teach Leafeon the very useful Razor Leaf for the cheap price of just one Heart Scale. Unless Leafeon is trained all the way to level 71, this will be Leafeon's only option to get a physical Grass move, and even if Leafeon does eventually learn Leaf Blade, it should still have Razor Leaf to rely on before it gets there.
There is a number of nifty TM options available, with X-Scissor being the main dish, offering useful Bug coverage against the fairly numerous late-game Psychic-types. It is a must have on any Leafeon. The rest is up to choice: Dig is pretty decent coverage for Electric-types, and Leafeon resists Electric moves, thus being a good Electric (and by extension, Volkner) buster; Return works well as a general purpose physical move, if Leafeon has nothing better to learn. Swords Dance is also available via TM, and works great against physically oriented enemies, as it will take them a while before they can deal much damage to Leafeon, for whom one setup turn is often enough to sweep. Bullet Seed can cover well until Leafeon reach Pastoria and get Razor Leaf, and Aerial Ace is also a nice middle game move.
Sadly, Seed Bomb is the only good tutor move Leafeon gets, and it is only available after the Champion.
Rock-types: Offering a resistance to four out of five of Leafeon's weaknesses, Rock-types are easily the best possible partners for Leafeon. The rare specially-based ones are even better, since they compensate for Leafeon's low special bulk.
Water-types: Water-types are easy to get and go very well with Grass-types in general, leaving very few types uncovered. Together with Rock-types and Leafeon, they compose a solid, compact core that the team can rely on, particularly if at least one of the three has good specially defensive capabilities.
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? Adamant is the best, but Naughty will also do, as Leafeon's special bulk will not be enough to actually take special hits. Whatever lowers Special Attack is automatically a good choice; even natures such as Impish, Careful and Jolly will work well. Brave and Relaxed are a mixed bag, since Leafeon's Speed is good, but it becomes borderline when nerfed. Anything that lowers Attack is terrible, and whatever penalizes Defense is also bad, so if Eevee has one of those natures just evolve it into something else.
At what point in the game should I be evolved? Waiting until level 29 is wise, as Eevee gets Bite by then, and Dark coverage helps. There is also the option of waiting until level 36 for Baton Pass, but given Sinnoh's threats, it is probably best if Leafeon is evolved as early as possible, as Eevee could die before reaching that level.
How good is Leafeon in a Nuzlocke? In a region like Sinnoh, it does not become useful in a huge lot of situations, but it does perform the bomb buster job well, and gets trainers rid of pesky Water/Ground-types as well as being capable of handling some of the Psychic threats in the late game. Most of its goodness comes from its Eeveelution status, however; for an Eeveelution, Leafeon is a little underwhelming, and there are better options to choose from.