In OU, because of Drizzle making rain's effects permanent and the Swift Swim + Drizzle ban, rain is primarily a defensive weather. In lower tiers, however, Rain Dance is almost always used offensively as you only have 7 turns to abuse its effects.
Down in NU, many powerful rain abusers reside. Rain is considered a very powerful play-style and is not to be taken lightly. There are various abusers and good setters to compliment rain... but there are also various checks!
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The two most common rain abusers are Gorebyss and Ludicolo.
Gorebyss, an incredibly threatening threat outside of rain, is even more dangerous with Swift Swim boosting its low Speed stat and increasing the power of its Water-type attacks. One of Gorebyss' biggest flaws is the lack of Speed before setting up; however, with rain up, this is not an issue. To top it off, a +2 Gorebyss in the rain is very hard to stop.
Ludicolo's Hydro Pump is very powerful under the rain. While its Special Attack is lower than Gorebyss and it lacks Hydro Pump, it's not inferior to Gorebyss. It has access to STAB Giga Drain allowing it to easily run through Water-types, along with better overall bulk and Speed. It also has the room to run Rain Dance if your other setters have fainted or if you have no time to bring them in.
Now, this is definitely not the end to the list of rain abusers; there are plenty more out there! Some other options include Floatziel and Seismitoad, as well as non Swift Swimmers such as Jynx, who can abuse the rain with her Dry Skin ability. What rain abusers do you like the best? Feel free to share... there are quite a lot of possibilities!
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Of course, there is more to rain than abusers... you also need Pokemon that can set-up rain. The best rain setter is the one that has recovery, great bulk, U-turn/Volt Switch/Explosion, resists Grass and Electric, and can support the team with other moves such as Stealth Rock, Heal Bell, and Healing Wish. What Pokemon can do all of that? None of them; which makes deciding the weather starter to use in the team makes using rain all the more fun! Some good weather setters include Ampharos, who is bulky, resists Electric, has Volt Switch, and can use Heal Bell; Gardevoir, who is bulky, has Taunt, is fast, and has Taunt and Healing Wish; and Miltank, who has Stealth Rock, Milk Drink, and great bulk and Speed.
Of course, that's not all. I've seen and used various other weather setters. Probopass, Electrode, Volbeat, and Rotom-S are also pretty good weather setters.
What is your personal favorite weather setter? Ampharos? Gardevoir? Miltank? Volbeat? Something not on the list? Why!
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Even if you don't like to use rain, it's still something that's in the meta -- you have to prepare for it! Some good weather checks include Gardevoir and Ludicolo, which can also be found on some rain teams themselves (especially Ludicolo). A lot of the time you have to resort to just stalling out the rain turns... or prevent the opponent from setting up rain in the first place! 7 turns may not be much, but it's enough for the opponent to potentially sweep you... unless you have anything to do about it.
How do you usually deal with rain? How much do you dedicate your team to it? What kind of weather checks do you use?
Another interesting thing to consider is what rain would be like without Damp Rock. Would dedicated rain teams disappear completely and only be used on ORD users such as Ludicolo (Offensive Rain Dance users)?