Sewaddle can only be found in the inside area of Pinwheel Forest in both games. Swadloon is also available there, but only in the dark grass; it can also be found on Route 6 and in Lostlorn Forest. Leavanny can also be found in the last two locations.
The much better version of Parasect, and prettier as well. Sewaddle is nearly broken in the early game, and difficult to fight against without the right coverage; after getting to Swadloon and then Leavanny status, it becomes a reliable ally for the middle portion of the game, though its severe weaknesses cripple it in a number of situations. For a Bug/Grass-type, it is still unbelievably useful in some battles, also thanks to its good bulk notwithstanding the weaknesses; nevertheless, whenever anything that reeks of birds or fire comes close, that becomes anything but Leavanny's job.
In general terms, keeping Leavanny alive throughout the journey is the real challenge, considering its weaknesses and the surprise coverage some Pokémon may have. If Leavanny does make it, though, it will prove itself even in surprising matchups - it can be bad against Pokémon whose STAB it resists, and then beat the likes of Shauntal's Chandelure if the right conditions are met. Unless Leavanny is expected to take on everything and send it to next Tuesday, which it obviously will not, it is highly likely to surprise even mildly experienced players with how well it uses its very limited resources.
Gym #3 - Burgh (Castelia City, Bug-type): Both Swadloon and Leavanny are at a disadvantage against Whirlipede due to Poison Tail, and Dwebble due to Smack Down; Dwebble's Smack Down is outdamaged by Razor Leaf, though, which makes the matchup possible for either Eviolite Swadloon or Leavanny. Both of them have a good chance against Burgh's Leavanny, provided that Swadloon is equipped with the Eviolite if it has not evolved yet; the ally Leavanny has Bug Bite, whilst Burgh's Leavanny's only Bug weapon is Struggle Bug. The matchup should be favourable, and not require any healing if Leavanny is already fully evolved, unless Burgh's own can outspeed it.
Bianca (Castelia City): An Eviolite Swadloon is still viable in this fight, though Leavanny will obviously have even less problems. Avoid any and all Fire-types, and hit the rest with Bug Bite, or Razor Leaf for the Water-types.
Cheren (Route 4): Do not fight Pidove, Pansear or Pignite. Everything else falls to either Bug Bite or Razor Leaf. Notably, Leavanny is a great candidate to fight Liepard, which is otherwise dangerous for a good few Pokémon at this point in the game. One thing to be mildly wary of is Dewott's Fury Cutter: it starts off being non-threatening despite the typing, but repeated misses with Razor Leaf may become problematic.
N (Nimbasa City): Sandile should be very easy, and Scraggy is doable, but confusion from Swagger must be healed since a self-hit in confusion from a Leavanny at +2 Attack is very damaging. After that, withdraw: neither Darumaka nor Sigilyph are suitable opponents for Leavanny.
Gym #4 - Elesa (Nimbasa City, Electric-type): No. Both of the Emolga have Aerial Ace and Zebstrika has Flame Charge. Leave them all alone.
Cheren (Route 5): His Pidove is evolved now, but the matchup has hardly changed any; Leavanny still needs to stay away from the Fire-types and the bird, while the Grass- and Water-types are easy for it, as well as Liepard. Once again, bear in mind the remote possibility of repeated Razor Leaf misses with Fury Cutter stacking up damage if Leavanny is fighting against Dewott.
Gym #5 - Clay (Driftveil City, Ground-type): Good matchup against Krokorok and Palpitoad, thanks to Leavanny's Razor Leaf and its resistances to their moves; Intimidate will prevent the Bug Bite OHKO against Krokorok, though. Excadrill is another story: not only it takes neutral damage from Grass, it also has Rock Slide and over-the-top Attack. Leavanny should not fight it.
Bianca (Driftveil City): Mostly the same matchup as before: the Grass- and Water-types are easy for Leavanny, as expected, and the now-evolved Musharna still falls in two turns to Bug Bite, unless it puts Leavanny to sleep with Hypnosis. Bring Awakenings just in case. Use Return against Herdier, which will be a 2-3HKO depending on Leavanny's stats and level (clean 2HKO before Intimidate); it does have Work Up, but Leavanny's Defense is good enough that anywhere before three boosts is not problematic for it.
N (Chargestone Cave): Everything here means trouble for Leavanny, starting from Boldore's Sturdy and Smack Down, to Ferroseed's Gyro Ball and Pin Missile, stopping by Joltik's Bug Bite and closing with Klink's defensive typing which is plain terrible for Bug and Grass Pokémon to fight against. Do not use Leavanny here. It can at best pull off a stall war against Klink, and it is not worth it unless the team has no other options, which should be available by now.
Gym #6 - Skyla (Mistralton City, Flying-type): It is a Flying gym, everything has Flying moves. One does not simply beat Flying Pokémon with Leavanny.
Cheren (Route 7): Liepard, Simisage and Servine are about as easy as they were before with their Bug weakness, as are their Water-type counterparts; the latter will be even easier if Leavanny has already learned Leaf Blade: it will no longer need to worry about missing Razor Leaf against Dewott. As for the Fire ones and the now-Unfezant, do not do it, that is not Leavanny's job.
Gym #7 - Brycen (Icirrus City, Ice-type): Again, a gym in which Leavanny is at a severe disadvantage, but it can still put in some work against one or two of Brycen's Pokémon. Vanillish cannot OHKO Leavanny in any way, not even with Frost Breath, and Leaf Blade 2HKOs it. Its very good Attack could also allow it to OHKO Cryogonal with Leaf Blade, but Cryogonal's Speed is higher than Leavanny's, meaning only higher-levelled specimens and/or those with a Speed-boosting nature should attempt it, and only after calculations: an average Leavanny will have a chance, but not a certainty, to actually OHKO. While Leavanny can take both Frost Breath and Aurora Beam, it cannot survive a critical Aurora Beam. Avoid Beartic, Leavanny has no way to beat it.
Bianca (Route 8): Fire-types must never be battled by Leavanny. Grass and Water are as easy as usual, or even easier, with Leavanny's recent move upgrades. Stoutland is a mixed bag: it has Work Up and is very powerful, plus Leavanny will find it hard to take it down if, once again, it leads the party; Intimidate dents its Attack too much, reducing Leaf Blade from a 3HKO to a 4HKO. A Leavanny with unscathed Attack, however, can manage a win against it.
Gym #8 - Iris/Drayden (Opelucid City, Dragon-type): Yet another gym with a conspicuous disadvantage for Leavanny. While the dragons' moves are not too threatening for its good Defense, Leavanny's own best weapon is X-Scissor, which will 2HKO Fraxure at best, and cannot do nearly as much against Druddigon and Haxorus. Fraxure and Haxorus also have Dragon Dance, and while this is not an issue if Fraxure can be 2HKOed, it definitely is in Haxorus' case, as its setup would spell bloodshed for Leavanny's entire team in the likely case that Leavanny would not KO it swiftly enough; this is further complicated by the healing items that Iris or Drayden can use. Druddigon has no stat-boosting moves, so Leavanny can attempt to fight it; be careful of Night Slash and its possible critical hits, especially in the event that Leavanny is already worn out from fighting Fraxure. This is especially relevant against Drayden, whose Druddigon has Rough Skin. If Leavanny is feeling brave, it can get off a few Swords Dances against Druddigon and then take down Haxorus with max-Attack Return, but that strategy might fail due to Druddigon's Dragon Tail. Using Substitute is an option, if Leavanny's Defense can keep it up: it prevents Dragon Tail from working, and Night Slash might do less than 25%, allowing Leavanny a free setup turn with each Substitute usage.
Cheren (Route 10): Unfezant and the Fire-types must still be left alone due to how badly they can hurt Leavanny, but everything else can be fought. If Leavanny is in low health when fighting Liepard, that could be a problem with its higher Speed and Night Slash; it is recommended to either heal or switch before Leavanny is hit.
Elite Four Shauntal (Pokémon League, Ghost-type): Cofagrigus is a great opportunity to set up Swords Dance; it has good Special Attack, but not over the top, and Leavanny's Special Defense is alright. Leavanny can take two Shadow Balls if neither of them is a critical hit and/or causes the Special Defense drop, thus achieving a +4 in Attack. The Miracle Seed is needed. Since Jellicent has Cursed Body, Leaf Blade should be used against it; at +4 in Attack, Shadow Claw OHKOs both Golurk and Chandelure, which means that even if Cursed Body disables Leaf Blade, Leavanny's sweeping plans will not be foiled.
Elite Four Marshal (Pokémon League, Fighting-type): Everything has Rock coverage - Stone Edge in most cases, except Mienshao that has Rock Slide - and good Defense at least, if not very good, and Sawk even has Sturdy as its ability. Do not do this with Leavanny.
Elite Four Grimsley (Pokémon League, Dark-type): A rather insidious fight. Scrafty has Poison Jab, which is normally a 3HKO, but it can 2HKO if it poisons at any given time; Liepard can both outspeed and has Aerial Ace, but can actually not OHKO unless it scores a critical hit, whereas Leavanny's X-Scissor is a certain kill. Krookodile is strong, but will go down to two Leaf Blade attacks even considering Intimidate (OHKOed without). Avoid Bisharp altogether, Leavanny just cannot fight it.
Elite Four Caitlin (Pokémon League, Psychic-type): Reuniclus offers an opportunity to set up a Swords Dance before sweeping with X-Scissor, but attempting more than one is by no means wise, as its Psychic hits like bricks. Leavanny will definitely tank a hit thanks to its not at all bad special bulk, but carefulness is needed nevertheless. X-Scissor should take care of the rest, and that also works for Gothitelle and Musharna: if Reuniclus was OHKOed by it - and after a Swords Dance, this is highly likely to be the case - then they will be OHKOed too, since Reuniclus is sturdier than them. Sigilyph, however, will end Leavanny's journey to greatness if it does not have Shadow Claw in its moveset, and even then Sigilyph is slightly faster, meaning only a Leavanny with considerable Speed / a Speed-boosting nature / higher levelled will be reasonably certain to not die to its Air Slash. This also assumes Swords Dance, and even Shadow Claw after Swords Dance is not a certain OHKO. Do not attempt to do this unless certain of success, or Leavanny will lose. In the event Leavanny is switched out against Sigilyph, and Gothitelle and Musharna are both alive, Leavanny can Swords Dance against either of them and then proceed to X-Scissor the rest, but it is safer to do so against Gothitelle than Musharna: the latter has Reflect and Charge Beam, which might both slow down the KO and trouble Leavanny with the Special Attack boosts, as Psychic will hit much harder then. The fight is doable, but Leavanny will need to take risks at some point for it to work out.
Reshiram/Zekrom (N's Castle): Reshiram has Fusion Flare, so do not even let Leavanny near it. Zekrom is different: as a mostly physical attacker, with "only" DragonBreath as a special move and thanks to Leavanny's resistance to Fusion Bolt, it is somewhat more doable with X-Scissor. Leavanny is only 3HKOed by Zekrom's strongest moves, and after a Swords Dance, it can 2HKO it. Critical hits on Zekrom's end, however, will put an end to Leavanny's life, so risk this only if there are no better options.
N (N's Castle): Everything in this team has super effective moves to hit Leavanny with, except Klinklang, which only has Steel and Electric coverage but is painful to take down with a Bug/Grass Pokémonì. Zekrom's Giga Impact can actually only 2HKO Leavanny, but a critical hit is a certain kill even from full health. As for the rest, Leavanny will OHKO Zoroark for certain if it manages to outspeed it and it has already been unmasked, but otherwise, it will die to its Flamethrower. Carracosta, while being doubly weak to Grass, has Sturdy, so Leavanny can only hope to score the KO if a teammate has set up entry hazards before the fight. The rest is unthinkable.
Ghetsis (N's Castle): If Leavanny can heal up the Toxic poisoning inflicted by Cofagrigus, it should be able to KO it. Providing it also does not take too much damage from Shadow Ball - which it should not, as its bulk is not that bad - Cofagrigus can also be used as setup material for Leavanny to Swords Dance against; like Shauntal's, Leavanny can take up to two hits from it, assuming no critical hits and no Special Defense drops. With a +4 boost, Leavanny is guaranteed OHKO everything on Ghetsis' team with either Leaf Blade or X-Scissor, whichever is more suitable for the type it fights against. However, if Shadow Ball scores the Special Defense drop, and/or Leavanny is otherwise threatened by Cofagrigus and prevented from setting up, there is nothing on Ghetsis' team that it can possibly fight, except Seismitoad; everything else has coverage against Grass and/or Bug, and will not be OHKOed by anything below two Swords Dances.
Post-Game: Leavanny will mostly be useful in its dedicated half of the League rematches, but not much so otherwise.
Sewaddle starts with Tackle, String Shot and Bug Bite. The latter is a powerful move for this point in the game, and is complemented even better by Razor Leaf, accessible at level 15. Right after evolving, at level 20, Swadloon learns Protect, and that is its only level-up move; naturally, this means it should be turned into a Leavanny as soon as possible, as even its level 1 options are not worth waiting for. Leavanny learns Struggle Bug at level 22, not really worth it as Bug Bite has long since been available, and then Slash at level 29, though at this point Return will also be already available and better. It then gets the useless Helping Hand at level 32 and the amazing Leaf Blade at level 36, very good and a welcome upgrade to the long-standing Razor Leaf. At level 39, another great upgrade comes in the form of X-Scissor; then there is Entrainment at level 43, not really useful, and the much better Swords Dance at level 46. Leavanny has many weaknesses, but also pretty good defensive stats, meaning it can make a good use of Swords Dance against opponents that do not have moves that can threaten it. Lastly, at level 50, it learns Leaf Storm, but this is only really better than Leaf Blade on a Modest Leavanny, or it can be run alongside it, but that means giving up on Return or Swords Dance.
The only noteworthy offensive TM moves Leavanny gets are Shadow Claw and Return. Return in particular is guaranteed to be a nuke, due to the nature of Leavanny's last evolution. Poison Jab and Aerial Ace are good options, but generally will not do just as well, and even Shadow Claw is outclassed by X-Scissor against everything but ghosts. Leavanny can also run support moves such as Reflect and Light Screen, both of which help its already good defensive power, but there is no saving it from its weaknesses; they can work for its teammates, though. Lastly, for lovers of the unusual, Dream Eater can be combined with GrassWhistle relearned from Swadloon at level 1, but it is unreliable at best, and by no means advised.
Rock-types: For Leavanny, a Rock-type is an essential companion. Rock-types cover both of Leavanny's double weaknesses both offensively and defensively, and they can also hit Bug- and Ice-types for super effective damage. Rock-types are not at all common in Unova, but the few that are offer great coverage.
Ground-types: A Ground-type will be a good complement for Leavanny if supported by a Rock-type as well. Rock and Ground are both weak to Grass and Water, but Leavanny has both types covered; Ground hits Poison and Rock for super effective damage as well, something Rock-types cannot do. Leavanny does have Grass moves, but it may be impossible for it to face some of the bulkier Rock-types, which Ground-types can easily deal with instead.
Water-types: As an alternative to Rock- or Ground-types, Water-types can work as Leavanny's teammates as well; they are also more common and easier to obtain. However, Water-types should not be on the same team as one Rock-type and one Ground-type, since the Grass weakness would become crippling in that case.
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.
Sewaddle Line Ratings
What Nature do I want? Adamant or Jolly. Special Attack in general will not be needed, so those natures work best. Impish and Careful also do, as they boost Leavanny's defenses and can come in handy against opponents that have no super effective moves against it.
Which Ability do I want? Swarm in general, as it will make Leavanny's attacks stronger in a pinch, while Chlorophyll itself tends to not be very useful: Leavanny will generally want to keep the move slot for something other than Sunny Day, and even if it did carry it, it would likely not have the time to set it up, nor would it be any useful as it can already outspeed most opponents on its own.
At what point in the game should I be evolved? Leavanny should be fully evolved before Burgh. It is not hard to evolve it fully, since its final evolutionary step is dependent on friendship, and it can defeat Burgh's Leavanny easily. Swadloon will do it too, but much more slowly.
How good is the Sewaddle line in a Nuzlocke? If it had an even marginally less unfortunate typing, it would be very good; alas, the many and heavy weaknesses it has make it just average overall. As many other specialised Pokémon, it is only good in a setting that favours its participation in battle, and said settings are not exceedingly common in Unova.
Weaknesses: Poison, Rock, Bug, Ice, Flying (x4), Fire (x4)