Need a Leavanny vs Leafeon vs Sawsbuck comparison? You know what, I'm going to put all this stuff here so I no longer have to dump it in my analysis. It'll be here. Prepare for a loooong post.
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First, I'll talk about
Sawsbuck. Sawsbuck is an amazing Pokémon on offensive teams as it has a variety of useful moves, and STAB on both Normal and Grass moves, which I consider quite useful. It also has some coverage moves like Nature Power (which also avoids Sucker Punch) and Megahorn.
Chlorophyll and Sap Sipper are awesome abilities, but I feel, with Base 95 being the current standard, that Sawsbuck has its main niche on sun teams, but it still competes with Leavanny and Leafeon even there as they have Chlorophyll as well.
It can also set up Swords Dance, but it has to utilize it itself, because if it runs Baton Pass it will be outclassed by Leafeon who has more bulk and raw power behind its Grass STAB.
This is also a flaw of Sawsbuck; Horn Leech is fairly weak and fails to 2HKO Alomomola as well as to OHKO Golurk if Sawsbuck has no boosts (Golurk can run 252 HP and 4 Def EV's, ensuring Jolly Sawsbuck can never OHKO Golurk.) Horn Leech does however, have the advantage of recovering some of Sawsbuck's health while attacking, so Life Orb and hazard residual damage does not hurt it as bad as it would otherwise.
This all being said, Sawsbuck is a good standalone sweeper, but its downside is very troublesome. Sawsbuck is very frail and as such has trouble finding time setting up if they can hit hard. Sawsbuck's frailty and lack of useful resists also means it has trouble utilizing Synthesis well. It can, however, break a wall or two.
In a nutshell, Sawsbuck can defeat the most threats because of its coverage, but is also the most frail. As a result,
Sawsbuck's niche is being a Swords Dance sweeper, preferably in the Sun.
Leafeon, is the physically bulkiest of the three. It also has the hardest hitting Grass STAB, but it has no secondary STAB to fall back upon and as such, will often run a Baton Pass set. Leafeon can easily Baton Pass Swords Dance boosts and has enough bulk to do it later again. It can either utilize Substitutes to pass along in the last moveslot, or Synthesis to recover. Note that with Substitute and Wish Leafeon will not utilize the Life Orb but Leftovers, and as a result, actually fail to have as much initial power as Leavanny, but as I said before, Leafeon has more chances to set up Swords Dance and to pass them than Leavanny has.
Wish can be passed alongside the Swords Dance boosts if you feel gutsy, but again if you are Wishpassing as well, you are in need of Leftovers or residual damage will stack on you. Also, Leafeon has low HP, making Wishpassing not as useful, especially as Leafeon rarely invests in HP. Leafeon prefers investing in Attack as well as Speed, as HP investment will detract from Attack even further, causing obvious power issues. As such, I feel Wish is not worth it on Leafeon.
Sweeping on its own is something Leafeon rarely does because it most likely runs only Leaf Blade, and even if it doesn't Drifblim and even Haunter wall it and smother it with their Super-Effective STAB moves. Leafeon's coverage moves, X-Scissor and Return, also don't provide much utility for it, and worse, Leavanny gets STAB on the former and Sawsbuck on the latter.
Leafeon has no movepool, as as an offensive supporter it is outclassed by Leavanny, whereas at a defensive supporter it is outclassed by half of the Grass-types in the tier, most notably Vileplume and Meganium but numerous others as well.
In a nutshell,
Leafeon's niche is running a Baton Pass set.
Leavanny also has its niches over the former two.
Leavanny's offensive niche is STAB X-Scissor alongside STAB Leaf Blade. This means, coming from Leavanny's Base 103 Atk, that Leavanny 2HKO's Musharna as well as Alomomola, and OHKO's Golurk. Whereas Sawsbuck can 2HKO Musharna with Megahorn, it fails to 2HKO Alomomola or OHKO Golurk. Leafeon fails to 2HKO Musharna with X-Scissor. Leavanny has many more coverage moves than Leafeon and actually a similar number compared with Sawsbuck, but however, these coverage moves are much weaker than Sawsbuck's, and all four miss out on Steel-types or useful coverage otherwise (Shadow Claw, Return, Aerial Ace, Poison Jab). However, Shadow Claw means that Leavanny can deal with Haunter and Drifblim unlike the other two.
That being said, Leavanny pulls off the offensive supporter (Leaf Blade + X-Scissor + Synthesis + Heal Bell / Magic Coat) better than Leafeon because of typing as well as STAB on X-Scissor. Leavanny also has slightly more overall bulk and certainly more bulk you're going to put to use due to the useful resists (and most notably, 2 ×4 resists against common types in NU), and lack of weaknesses to priority moves (outside of Ice Shard, but Piloswine is the only notable user of that move).
Sawsbuck is too frail to pull this off as well as that it misses out on Heal Bell. Other Bugs in NU have no access to Synthesis, Magic Coat and Heal Bell, allowing Leavanny to differentiate itself from its fellow Bugs.
Leavanny does however have some problems especially when it opts to not to run these moves, and those are missing out on Base 95 speed as well as no significant bulk, but however, Leavanny's typing allows it to set up boosting moves like Swords Dance on a myriad of defensive threats. Leavanny however, misses out on useful coverage outside of Shadow Claw and Sawsbuck's STAB on Return, and as such, Sawsbuck outclasses it as a Swords Dance sweeper unless you are really desperate for STAB X-Scissor (but note that Sawsbuck has Megahorn, and as such can run Grass/Bug/Normal coverage as well). It misses out on the physical bulk of Leafeon as well as having a Stealth Rock weakness, and as such is outclassed as a Baton Passer.
As a Bug-type Swords Dance sweeper, Leavanny is outclassed by Pinsir because Pinsir has moves like Earthquake, Stone Edge and Close Combat, thouth Leavanny also has a small advantage; unlike Pinsir, Leavanny can OHKO Alomomola at +2.
One thing is not touched upon, and that is the fact Leavanny can be a decent Agility sweeper as it hits just hard enough to not need any boosts in power (ensure hazards are up though, as if it would be a Riolu), utilizing the obvious Agility and Dual STAB and a coverage move of choice, most likely Shadow Claw (If you go for Baton Pass, you are reaching Ninjask territory).
In short,
Leavanny's niche is running an offensive support set.
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Now, where to put them? There are numerous problems these three Grass-type Pokémon share and as such aren't mentioned before; neither of them gets past Torkoal, Weezing or Golbat. Also they all have problems to be standalone sweepers, as they all have numerous checks and counters. Sawsbuck has the least amount of checks and counters but the others can do their job even if they are still in play.
They share Chlorophyll, making the decision not any easier if you opt to run weather.
As standalone sweepers, Torterra might be better than those three as well considering it has significant bulk, EdgeQuake as well as Wood Hammer, and it hits pretty damn hard.
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I think I am done with my rant. Did you read it? Good. I am still not done with my post.
About
Tropius: SubSeed + Protect + Air Slash and SitrusHarvest. Is that niche good enough for C-rank? I am not so sure, but I can tell from my experience how annoying this thing is to face. Especially if you don't have Ice attacks - and sometimes even if you have.
