With fantastic Speed, solid Attack, and a great ability, Swellow makes its mark in RU as one of the premier late-game sweepers in the tier. Swellow's gives it the edge against Pokemon such as Sceptile, Choice Scarf Emboar, and Cinccino, three big threats in RU that few other Pokemon can claim to outspeed. Swellow also has a great ability in Guts, which boosts its decent Attack to very high levels when afflicted with a status condition. Two high-powered STAB moves and priority in Quick Attack only augment this firepower and cleaning ability, making it quite difficult to stop Swellow with sheer bulk or Speed once it gets going.
In spite of all of these good qualities, Swellow has next to no coverage against the numerous Rock- and Steel-type Pokemon in RU. It doesn't help that Steelix and Rhydon are two of the premier physical walls in the tier, as both can handle Swellow without much difficulty. With paltry defenses, a weakness to Stealth Rock, the use of a Toxic Orb to activate Guts, and recoil from possible Brave Birds, Swellow's lifespan is very short and generally relegates it to the late-game sweeping role. In spite of this, Swellow is very good at what it does and makes for a great a late-game cleaner with the right support.
This is Swellow's best set to run in RU and lets it perform as a great late-game cleaner. This set aims to activate Swellow's Guts ability to power up its moves and go on a late-game sweep. Facade should be the move you use under most circumstances. After Guts is activated, Facade has an incredible 210 Base Power after STAB, allowing it to get through even frailer resistances such as Archeops and Aerodactyl. Brave Bird is useful for hitting defensive Grass-types such as Tangrowth, Torterra, and Amoonguss as well as Pokemon that resist Facade like Escavalier, Durant, and Spiritomb. U-turn lets Swellow hit its checks and counters as they come in and switch to a teammate to either take them out or force them out to take more entry hazard damage. The final move is mostly used to ensure Swellow can activate Guts safely. Quick Attack has the more general use as it allows Swellow to revenge kill weakened threats while activating Guts. It's also useful to get in front of other priority users such as Absol, which might be sent in to revenge kill Swellow with its STAB Sucker Punch. Protect is an option to ensure Guts is activated safely. However, keep in mind that once the Toxic Orb is activated, Swellow takes poison damage every turn, which makes it somewhat counter-productive to scout for moves later with Protect.Team Options & Additional Comments >>>
Quite frankly, Swellow has an incredibly limited movepool. However, there are a couple of options you could consider. A Choice Band set seems decent thanks to Guts allowing Swellow to not be crippled by burns and it having a strong STAB move in Brave Bird. However, Braviary not only has a much higher Attack stat but also has the ability to overwhelm opposing Rock- and Steel-types with Superpower. Pursuit can catch fleeing Ghost- and Psychic-types, but Swellow is better off just firing off one of its STAB moves. Toxic is an interesting option to cripple Rhydon for the remainder of the match. Steel Wing can catch Rhydon for some damage, but its low Base Power and Rhydon's insane physical bulk makes this a lesser option. Finally, Endeavor can be used as a last-ditch effort to damage an opposing threat if Swellow is really low on HP and can't finish off a sweep.
Checks and Counters
Steelix and Rhydon are, bar none, the best counters to Swellow. With a resistance to both STAB moves and very high physical bulk, both can take numerous attacks from Swellow and respond with Gyro Ball or Rock Blast respectively. Aggron is the most notable offensive counter to Swellow as it sports a 4x resistance to both of Swellow's STABs and has a powerful Head Smash to decimate Swellow or any teammate that tries to switch in. Choice Scarf Rotom is incredibly annoying for Swellow as it's immune to Facade and Quick Attack and has STAB Volt Switch to build momentum or outright KO Swellow if it stays in. Choice Scarf Rotom-C is also problematic, although a weakened one will have trouble dealing with Quick Attack. Aerodactyl and Accelgor are two of the very few Pokemon that naturally outspeed Swellow, but Accelgor's frailty makes it hard for it to deal with Quick Attack.
Priority users are always an option to revenge kill Swellow, but keep in mind that most will have to deal with Quick Attack first as Swellow outspeeds them. Entei is one of the two exceptions as ExtremeSpeed's +2 priority lets it hit before Swellow can move. The other exception is Spiritomb as its Ghost typing makes it immune to Quick Attack. Samurott and Kabutops can use Aqua Jet and Kabutops even resists Quick Attack. Hitmonchan and Hitmonlee can use Mach Punch. Hitmonchan's is boosted by Iron Fist, and Hitmonlee's is usually backed by a Fighting Gem. Although it will have difficulty tanking it, Ferroseed can force Swellow to take a ridiculous amount of damage from using Brave Bird due to Iron Barbs and the recoil incurred from using Brave Bird in the first place. Pokemon with high physical bulk such as Alomomola and Uxie can take a hit and strike back with their STAB moves, but both must be careful about taking too much prior damage, especially Uxie as Alomomola has Wish and Regenerator to stay in good health throughout the match.