Overview
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Tornadus's niche rests in its powerful ability Prankster, which grants priority to all of its non-attacking moves. This, combined with its high Special Attack and Speed, and access to a powerful STAB Hurricane, makes it an ideal supporter for rain teams. While it can set up a Prankster Rain Dance to help the rest of the team, it is also a threatening and powerful Pokemon in rain. However, this is where Tornadus's good points end. Mediocre defensive typing and bulk make it difficult to both set up rain and take more than one or two hits. It also faces great competition from its Therian forme, which is faster and has Regenerator, also a great ability. Tornadus is less threatening without permanent rain, but underestimating it would be a mistake, as it can be a powerful member of rain teams.
Rain Dance
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name: Rain Dance
move 1: Rain Dance
move 2: Hurricane
move 3: Superpower / Focus Blast
move 4: Knock Off / Taunt
ability: Prankster
item: Life Orb / Damp Rock
evs: 4 Atk / 252 SpA / 252 Spe
nature: Naive / Timid
Moves
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Rain Dance makes Tornadus an auxiliary weather inducer for rain teams, an important role with the lack of permanent rain. Hurricane is Tornadus's main offensive move, with great power and neutral coverage. Superpower gets good coverage with Hurricane and has 100% accuracy, but Focus Blast is a stronger option with that provides the same coverage. Knock Off hits Aegislash, while Prankster Taunt is a great disrupting move. U-turn can also be used to possibly get a rain sweeper in for free, possibly for free,(RC) if you want to preserve Tornadus for later.
Set Details
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Special Attack and Speed are maximized so that Tornadus hits as hard as possible while taking advantage of its great Speed tier that puts it ahead of Pokemon like Latios. Life Orb is the best item because 5 turns of rain is usually enough,(RC) and Tornadus appreciates the wallbreaking power, but Damp Rock can be used to maximize rain turns if you desire. Prankster is the ability to make Rain Dance near guaranteed The Prankster ability almost guarantees setting up Rain Dance. If going mixed, a Naive nature is better so that Tornadus can take physical Bug- and Fighting-type moves better, but Timid should be used if you opt for Focus Blast and Taunt.
Usage Tips
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Tornadus should come in mid-game (add hyphen) after the first time your rain runs out. It Tornadus should not be your first rain setter, as this squanders its offensive capabilities. After setting up Rain Dance, it's probably best for it to just spam Hurricane or the appropriate coverage move until it's KOed, which should be soon, to let another rain sweeper in for free. However, if you see the opportunity to switch in another Pokemon for free, then you can save Tornadus and get another set of off another guaranteed rain later.
Team Options
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Deoxys-S's Damp Rock set makes a great rain lead to set the pressure. Politoed is also basically mandatory for rain teams, thanks to setting up rain just for by switching in. After that, rain abusers should be added. Electric-types such as Thundurus, Thundurus-T, Raikou, and Mega Manectric are good Pokemon that take advantage of Thunder's 100% accuracy in rain. Swift Swimmers are also a crucial part of rain teams. Kabutops and Kingdra are both good choices, while Ludicolo, Omastar, and Seismitoad are more unconventional choices that can still be effective.
Other Options
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Tornadus doesn't have many other usable options in OU. Tailwind is a very notable option,(RC) though, as it gives all Pokemon on Tornadus's team doubled Speed for 3 turns, and is near guaranteed with Prankster, but the set is overall inferior to Rain Dance because Tornadus is much less threatening outside of rain, and Rain Dance will double the Speed of Swift Swimmers anyway. A physical Defiant set with no item and Acrobatics can work as an all-out attacker and Defog abuser, but it is too weak without an item.
Checks & Counters
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**Electric-types**: Electric-types such as Thundurus and Rotom-W can easily survive Tornadus's attacks and hit it with a STAB Thunderbolt or Volt Switch.
**Weather-inducers**: Mega Charizard Y's sun reduces Hurricane's accuracy to 50% and can take Tornadus out with ease. Scarf Tyranitar can also change the weather, while outspeeding and KOing.
**Bulky Flying-resists (add hyphen)**: Specially Defensive Skarmory can avoid a 2HKO from any attack that Tornadus will carry. Aegislash also takes even Knock Off fairly well, and can help protect itself against that with King's Shield.
With Tornadus, the real threat is its rain support for the rest of the team, which is much harder to stop than Hurricane.