[OVERVIEW]
While not necessarily the best Swift Swim sweeper for a rain team in the OU metagame, Omastar is unarguably the strongest, boasting the highest Special Attack and possessing one of the best setup moves in the game, Shell Smash. These traits allow it to fire off truly devastating Hydro Vortexes with a Z-Crystal. With a Shell Smash boost, even the bulkiest of Water-resistant Pokemon, such as Assault Vest Tangrowth, Amoonguss, and Ferrothorn, must be completely healthy to have a hope of surviving Omastar's Hydro Vortex. Even Chansey, one of the best blanket checks to special attackers in the metagame, has difficulty handling Omastar. Omastar is a premier breaker for rain teams, as it is great at demolishing checks for its fellow rain attackers like Kingdra and Ash-Greninja.
However, unlike Kingdra and Mega Swampert, Omastar is unable to outspeed common Choice Scarf users like Keldeo and Garchomp with just the Swift Swim boost, and it even falls short of very fast unboosted Pokemon like Mega Alakazam. For it to have any hope of sweeping on its own, it usually must acquire a Shell Smash boost, which is often easier said than done. Furthermore, due to its poor Special Defense, it is vulnerable to opposing Water-type attacks, most notably Ash-Greninja's Water Shuriken. For this reason, Omastar pairs best with Kingdra, which has a double resistance to Water-type attacks that is invaluable for taking rain-boosted Water Shurikens.
[SET]
name: Waterium Z
move 1: Shell Smash
move 2: Hydro Pump
move 3: Earth Power
move 4: Ice Beam / Ancient Power
item: Waterium Z
ability: Swift Swim
nature: Modest
evs: 4 Def / 252 SpA / 252 Spe
[SET COMMENTS]
Moves
========
Shell Smash is Omastar's most important move, as it gives Omastar the speed to outspeed all Choice Scarf users and the power to blast through bulky Water-resistant targets. Hydro Pump is Omastar's strongest Water-type attack, and its shaky accuracy is less of a problem when it can be used as a Z-Move. Earth Power is used for hitting Toxapex and gives a 100% accurate option against targets like Heatran and Gengar. Ice Beam is primarily for Grass-type targets like Tangrowth and Amoonguss and allows Omastar to conserve its Z-Move against them; it is also helpful against targets like Zygarde and Garchomp. Finally, Ancient Power allows Omastar to hit Mantine and gives it a 100% accurate option against Mega Charizard Y, but it does little else.
Set Details
========
Omastar needs all the power and Speed it can get, thus maximum Special Attack and Speed EVs are used in combination with a Modest nature. Swift Swim is essential to performing on a rain team, and Waterium Z allows Hydro Pump to be used as a Z-Move.
Usage Tips
========
Omastar relies heavily on rain being up, so prioritize keeping it up for Omastar and its teammates. Omastar is best used not for sweeping but for blowing away checks to the Water-type attackers it's best paired with. Shell Smash is key to getting the power and speed to truly threaten a team, but often Omastar won't be able to get the boost for free, so you must weigh the risk and reward of using Shell Smash or just firing off an attack. Since many targets that Omastar is supposed to be able to break have Giga Drain, getting a KO is important or else the target can heal as much as a third of their health back while KOing Omastar.
Team Options
========
Omastar is only viable on a rain team, thus Pokemon essential to rain teams are obvious teammates. Pelipper has access to Drizzle and should always be paired with Omastar. Mega Swampert and Kingdra are both very threatening Swift Swim sweepers; however, Kingdra pairs with Omastar better, since its double resistance to Water-type attacks is valuable for handling Ash-Greninja. Ash-Greninja is also quite viable on rain teams and can serve as a fellow Water-type attacker. An Electric-type attacker is a good pick for rain teams due to their powerful, 100% accurate Thunders under rain; Tapu Koko often fills this role. Ferrothorn is a good utility Pokemon on rain teams due to its ability to lay entry hazards and handle opposing Water-type attackers. Ferrothorn is also a tough Pokemon for rain teams to handle, so they must have a good answer to it; Mega Heracross is a seriously threatening Pokemon that handles Ferrothorn effortlessly, and Keldeo also works to threaten Ferrothorn, take advantage of the rain, and check Ash-Greninja.
[STRATEGY COMMENTS]
Other Options
=============
Surf is a 100% accurate, less risky Water-type STAB move, but it's not as valuable because Hydro Pump can be used as a more powerful Z-Move; the power drop is quite substantial. Hidden Power Grass hits Gastrodon but nothing else. Life Orb can be used over Waterium Z if your team's Z-Move is used elsewhere, but at that point you're better off not using Omastar. Omastar can also accomplish an entry hazard lead set on non-rain teams, laying Spikes and / or Toxic Spikes while getting boosted Speed from Weak Armor to usually guarantee two layers; this set can also use Scald and Knock Off to annoy the opposing team.
Checks and Counters
===================
**Bulky Grass-types**: If Hydro Vortex has already been used, bulky Grass-types such as Tangrowth, Tapu Bulu, Ferrothorn, and Amoonguss can check Omastar, but they still take a lot of damage from Ice Beam.
**Bulky Water-types**: If Hydro Vortex has been used, bulky Water-types can beat Omastar with rain-boosted Scalds. Good choices are Toxapex and Tapu Fini, although Toxapex must watch out for Earth Power. Gastrodon is an especially good answer because it has an immunity to Water-type moves, and it beats any Omastar that does not have Hidden Power Grass. Mantine is similarly a good check, since it is also immune to Water-type moves, but it must watch out for Ancient Power.
**Ash-Greninja**: Choice Specs- and rain-boosted Water Shuriken KOes Omastar if it lands at least three times in Greninja's base form and always if it has activated Battle Bond.
**Weather Setters**: Mega Charizard Y, Tyranitar, Mega Tyranitar, and Alolan Ninetales all beat unboosted Omastar by removing the rain with their abilities. None really like switching into Omastar, however.
**Chansey**: Chansey is able to beat Omastar one on one if Hydro Vortex has already been used, but otherwise, Omastar can use Shell Smash twice and OHKO Chansey with Hydro Vortex.
While not necessarily the best Swift Swim sweeper for a rain team in the OU metagame, Omastar is unarguably the strongest, boasting the highest Special Attack and possessing one of the best setup moves in the game, Shell Smash. These traits allow it to fire off truly devastating Hydro Vortexes with a Z-Crystal. With a Shell Smash boost, even the bulkiest of Water-resistant Pokemon, such as Assault Vest Tangrowth, Amoonguss, and Ferrothorn, must be completely healthy to have a hope of surviving Omastar's Hydro Vortex. Even Chansey, one of the best blanket checks to special attackers in the metagame, has difficulty handling Omastar. Omastar is a premier breaker for rain teams, as it is great at demolishing checks for its fellow rain attackers like Kingdra and Ash-Greninja.
However, unlike Kingdra and Mega Swampert, Omastar is unable to outspeed common Choice Scarf users like Keldeo and Garchomp with just the Swift Swim boost, and it even falls short of very fast unboosted Pokemon like Mega Alakazam. For it to have any hope of sweeping on its own, it usually must acquire a Shell Smash boost, which is often easier said than done. Furthermore, due to its poor Special Defense, it is vulnerable to opposing Water-type attacks, most notably Ash-Greninja's Water Shuriken. For this reason, Omastar pairs best with Kingdra, which has a double resistance to Water-type attacks that is invaluable for taking rain-boosted Water Shurikens.
[SET]
name: Waterium Z
move 1: Shell Smash
move 2: Hydro Pump
move 3: Earth Power
move 4: Ice Beam / Ancient Power
item: Waterium Z
ability: Swift Swim
nature: Modest
evs: 4 Def / 252 SpA / 252 Spe
[SET COMMENTS]
Moves
========
Shell Smash is Omastar's most important move, as it gives Omastar the speed to outspeed all Choice Scarf users and the power to blast through bulky Water-resistant targets. Hydro Pump is Omastar's strongest Water-type attack, and its shaky accuracy is less of a problem when it can be used as a Z-Move. Earth Power is used for hitting Toxapex and gives a 100% accurate option against targets like Heatran and Gengar. Ice Beam is primarily for Grass-type targets like Tangrowth and Amoonguss and allows Omastar to conserve its Z-Move against them; it is also helpful against targets like Zygarde and Garchomp. Finally, Ancient Power allows Omastar to hit Mantine and gives it a 100% accurate option against Mega Charizard Y, but it does little else.
Set Details
========
Omastar needs all the power and Speed it can get, thus maximum Special Attack and Speed EVs are used in combination with a Modest nature. Swift Swim is essential to performing on a rain team, and Waterium Z allows Hydro Pump to be used as a Z-Move.
Usage Tips
========
Omastar relies heavily on rain being up, so prioritize keeping it up for Omastar and its teammates. Omastar is best used not for sweeping but for blowing away checks to the Water-type attackers it's best paired with. Shell Smash is key to getting the power and speed to truly threaten a team, but often Omastar won't be able to get the boost for free, so you must weigh the risk and reward of using Shell Smash or just firing off an attack. Since many targets that Omastar is supposed to be able to break have Giga Drain, getting a KO is important or else the target can heal as much as a third of their health back while KOing Omastar.
Team Options
========
Omastar is only viable on a rain team, thus Pokemon essential to rain teams are obvious teammates. Pelipper has access to Drizzle and should always be paired with Omastar. Mega Swampert and Kingdra are both very threatening Swift Swim sweepers; however, Kingdra pairs with Omastar better, since its double resistance to Water-type attacks is valuable for handling Ash-Greninja. Ash-Greninja is also quite viable on rain teams and can serve as a fellow Water-type attacker. An Electric-type attacker is a good pick for rain teams due to their powerful, 100% accurate Thunders under rain; Tapu Koko often fills this role. Ferrothorn is a good utility Pokemon on rain teams due to its ability to lay entry hazards and handle opposing Water-type attackers. Ferrothorn is also a tough Pokemon for rain teams to handle, so they must have a good answer to it; Mega Heracross is a seriously threatening Pokemon that handles Ferrothorn effortlessly, and Keldeo also works to threaten Ferrothorn, take advantage of the rain, and check Ash-Greninja.
[STRATEGY COMMENTS]
Other Options
=============
Surf is a 100% accurate, less risky Water-type STAB move, but it's not as valuable because Hydro Pump can be used as a more powerful Z-Move; the power drop is quite substantial. Hidden Power Grass hits Gastrodon but nothing else. Life Orb can be used over Waterium Z if your team's Z-Move is used elsewhere, but at that point you're better off not using Omastar. Omastar can also accomplish an entry hazard lead set on non-rain teams, laying Spikes and / or Toxic Spikes while getting boosted Speed from Weak Armor to usually guarantee two layers; this set can also use Scald and Knock Off to annoy the opposing team.
Checks and Counters
===================
**Bulky Grass-types**: If Hydro Vortex has already been used, bulky Grass-types such as Tangrowth, Tapu Bulu, Ferrothorn, and Amoonguss can check Omastar, but they still take a lot of damage from Ice Beam.
**Bulky Water-types**: If Hydro Vortex has been used, bulky Water-types can beat Omastar with rain-boosted Scalds. Good choices are Toxapex and Tapu Fini, although Toxapex must watch out for Earth Power. Gastrodon is an especially good answer because it has an immunity to Water-type moves, and it beats any Omastar that does not have Hidden Power Grass. Mantine is similarly a good check, since it is also immune to Water-type moves, but it must watch out for Ancient Power.
**Ash-Greninja**: Choice Specs- and rain-boosted Water Shuriken KOes Omastar if it lands at least three times in Greninja's base form and always if it has activated Battle Bond.
**Weather Setters**: Mega Charizard Y, Tyranitar, Mega Tyranitar, and Alolan Ninetales all beat unboosted Omastar by removing the rain with their abilities. None really like switching into Omastar, however.
**Chansey**: Chansey is able to beat Omastar one on one if Hydro Vortex has already been used, but otherwise, Omastar can use Shell Smash twice and OHKO Chansey with Hydro Vortex.
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