(I) Introduction
Rain is one of the most meta-defining and memorable archetypes of Competitive Pokémon across generations, especially in later ones.
In ADV, however, it has never seen a lot of use, and because of the latter, it can be described as a relatively unexplored archetype.
One might think “Why use Rain in ADV? I mean, I have a lot of solid options regarding team options. What does Rain do better? Versus whom? How do I even begin to build a Rain team?”.
All of these are valid questions that I’m going to try my hardest to answer in this thread, because I don’t use Rain when I’m bored or to mess around on the ladder; I play Rain when I want to win games. I want players to consider it a viable playstyle with a place in the meta and as a force to be reckoned with.
(II) Team Structure
Rain Offense teams are divided in three main playstyles (or sub-archetypes). I will denote the main roles Pokémon have in Rain teams and then use that notation to analyze each sub-archetype more deeply.
[Special Wall Bait and/or Remover] (SpBait) (SpWRemover)
These Pokémon accomplish, arguably, the most important role in Rain Offense teams: removing Special Walls. They accomplish this either by baiting them or by checking them and threatening Explosion.
Gengar:

Baits all special walls in the game bar waters. Super important member in Rain teams. Speed control. Offensive Presence. Utility. Valuable resistances/immunities.
Sets - https://pokepast.es/1ebfe12f7df41078
Zapdos:

Baits all special walls in the game bar waters. Baton Pass to trapper. Speed control (Timid). Offensive Presence. Utility. Can pivot really well.
Sets - https://pokepast.es/4148c522ff654015
Dugtrio:

Key piece of Rain teams thanks to its ability to trap and remove a extensive range of threats, especially Blissey. Revenge Killer. Speed control. Electric immunity.
Pseudo-rock resist.
Sets - https://pokepast.es/4424f0e53024f74d
Metagross:

Offensive enabler. Really good at pressuring Blissey and Snorlax. Its objective in Rain teams as Special Wall remover is to boom on Milotic/Suicune.
Valuable resistances/immunities.
Sets - https://pokepast.es/16d344383882ef65
Niche mons that can also work as Special Wall removers:
Regice

Jynx
Umbreon
Wailord
Qwilfish
[Note - I’m still working on spreads; I’ll add Niche Pokémon movesets in the future.]
[Special Wall Abusers] (SpWAbuser)
This Pokémon do not try to remove Special Walls but rather use their presence and passive nature to set up on them. This is usually done with 101 HP Substitutes and Calm Mind or with Pokémon that pressure common Special Walls with powerful physical attacks.
Suicune:

Really solid addition to any type of Rain Offense team thanks to its ability to set-up in front of Blissey and Milotic without sand.
Will almost always provide value over a match (sometimes it can even become your main win condition). Valuable resistances/bulk. Pseudo-DD check.
Sets - Substitute, Roar, HclatCune
Jirachi:

Set-ups on most “rain counters”. Natural bulk. Rock resist. Valuable resistances/immunities.
Sets - Substitute, Wish
Snorlax:

Checks Blissey, Milotic, Regice. Special sponge. Can boom on threats. Removes Skarm/Tar/Meta depending on set.
It can become a win condition with Curse set thanks to weather support.
Sets - BaitLax, Curse
Tyranitar:

It can chip Suicune and Milotic. Checks Snorlax. Can pressure and set-up on Bliss thanks to Lum Berry, Sandstorm helps in stall Matchup before Rain Dance.
Secondary win condition on late-game. Valuable resistances/immunities. Helps with Zapdos and, to a lesser extend, Jolteon.
Sets - Dragon Dance
[Rain Sweepers] (RainSweeper)
Rain sweepers are what make this thread’s title “Rain Offense” and not just “Special Offense”. These Swift Swim blessed Pokémon will demolish most offensive teams, and thanks to extensive team building support, break through defensive teams as well.
Kingdra:

Default and most consistent rain sweeper. Decent unboosted speed tier.
Good defensive typing (only one weakness) and solid special attack with good coverage makes Kingdra super dangerous in Rain.
Sets - Standard, EndPetaya
Ludicolo:

Overall less bulky and worst typing than Kingdra (and thus, less consistent as a set up sweeper). It has Grass STAB which is really helpful vs Suicune/Milotic.
Sets - Standard
Omastar:

Hardest hitter rain sweeper. Normal resist is nice to have but really bad typing overall. Pseudo-spiker.
Sets - Standard, Spikes
Rain Sweepers spreads and movesets: https://pokepast.es/5206843588fbf7be
Niche mons that can also work as Rain Sweepers:
Gorebyss
Mantine
[Special Attackers] (SpAttacker)
Most of the time, as we will see in team structure sub-sections, Rain Sweepers aren’t alone in rain’s offensive core. Special Attackers also greatly benefit from having Special Walls removed from the match. Additionally, they usually add synergies and soft-checks to threats that otherwise can be problematic to a standard Rain Offense team.
Jolteon:

Works similar to Zapdos. Speed control. Helps with the general electric weakness that rain teams naturally have.
Thunder Wave is useful to set up a rain sweep and can clean in late-game if no special walls are present.
Starmie:

Technically speaking Offensive Starmie can be viewed as a “worst” (deals less damage and is slower) rain sweeper that doesn’t need rain to work, which is amazing.
It can easily clean offensive teams once Snorlax/Regice is removed. Speed control. Spinner.
Alakazam:

A “better and worse” Gengar. Better offensively; higher SpA and Speed, and access to Calm Mind. Worse in every other aspect.
Really good with no special walls/Tyranitar to stop it.
Raikou:

A slower, bp-less, Jolteon. It can set-up on Blissey with weather support and pressures bulky waters. Endure sets become a possibility without Sand too.
[Glue] (Glue)
Lastly we have the valuable Pokémon known as “glue mons”. These kind of Pokémon fit well in the majority of variations of an archetype and patch holes offensively and defensively. The best glue Pokémon for Rain Offense are listed here:
Swampert
Jirachi
Metagross
Claydol
[Note - I'll probably expand this section of the thread more in the future. I'm planning to add more spreads and using Spoiler format instead of Pokepaste.]
II.a) Special Offense - “Bait, remove, click Rain Dance”.
This kind of Rain Offense playstyle is recognizable for using more Special Wall abusers than Removers or Baiters. It usually runs 2 Rain Sweepers. It also has a similar team structure to CM Spam teams.
Structure:
(SpWBait) + 2 (SpWRemover) + 2 (SpWAbuser) + 2 (RainSweeper)
Example lineups:
II.b) Spike Stack - “Spike up, pressure your opponent, clean”.
Second on the list we have Spike Stack + Rain. This archetype is, play-wise and structure-wise, similar to “StarJolt” teams. Its main difference from other rain teams is the presence of Spikes; these act as an additional method of pressuring special walls and limiting opponent’s options. Usually these kind of teams don’t have any defensive backbone, so they have to be played very aggressively but also have a great risk/reward ratio.
Structure:
(Spiker) + (SpWBait) + (SpWRemover) + (SpWAbuser) + (RainSweeper) + (Glue)
Example lineups:
II.c) Spikeless Offense - “Attack, set up, sweep”.
Similar to Rain Special Offense but more focused in removing special walls using “brute-force” and opening windows to set up a psychical threat. It uses its rain sweeper as a sort of “last option”. Usually has 2 Removers, 1 or 2 Physical Sweepers and 1 Rain Sweeper.
Structure:
(SWBait) + 2 (SWRemover) + 2 (SWAbuser) + (RainSweeper) + (Glue)
Example lineups:
[Note - I’m still working on lineups; I’ll probably add more in the future, as well as a little team dump.]
(III) Matchup
Here I will talk briefly about matchups, mainly because they are really lineup/moveset dependent. Your rain team will have a harder time dealing with Snorlax if you don’t have Metagross or Tyranitar and a harder time versus Milotic if you don’t have Zapdos or Jolteon.
One thing worth noting about Rain Offense, matchup wise, is that it isn’t as polarizing as one might think at first glance. It has a relatively good match-up versus most archetypes.
III.a) Rain vs Offense
Standard (Mixed/Physical) Offense
Standard Offense teams are your best matchup by far. Outside of a few specific Pokémon (such as Snorlax, Regice, and Sword Dance Celebi) your rain sweepers will OHKO everything.
Gameplan: In most matches vs Offense your game plan is trying to gain momentum and “chipping” their water switch-ins. Use Explosion’s reset turn effect to set up a Rain Dance turn more freely. Dugtrio should be played carefully because it can give Flying-type DDers free turns to boost.
CMSpam
CMSpam is the hardest offensive matchup for Rain. Most common Pokémon used in CMSpam can soft-check Rain Sweepers and even set up on them.
Gameplan: You need to prioritize trapping Celebi. CMSpam teams usually have Porygon2 so you can’t risk using Dugtrio earlier. Pokémon like Agility Metagross, Thunder Wave Jolteon, and Snorlax can help checking CMers.
III.b) Rain vs Stall
Stall matchup can range from favorable to almost impossible. The main variable at determining matchup difficulty when facing Stall is the quantity, and combination, of dedicated special walls.
Suicune + Blissey/Snorlax - This is usually the most common pair of special walls in “MagDol stall” builds. Given their commonness, most Rain builds have (need to have) the amount of removal resources required to remove/pressure them.
How to break? Suicune and Blissey usually have to be “boomed” on, while Snorlax can be pressured and chipped down to the point where Hydro Pump 2HKOs (60%).
Celebi + Blissey + Starmie/Milotic - This is the hardest defensive core to break with Rain. If the opponent is using Starmie, Spike Stack Rain is almost an impossible matchup.
How to break? Adamant Bulky Dugtrio is key to break these type of cores, as you can kill Blissey consistently and then chip Celebi super hard with HP Bug later in the game. If you are using Jolly Dugtrio, you can trap Starmie, chip Blissey/Celebi and then use one of your Substitute Special Attacker Pokémon to set up on their weakened Special Walls.
III.c) Biggest threats (Danger level)
Milotic
Celebi
Suicune
Jolteon
Snorlax
Zapdos
Blissey
(IV) Final thoughts
As a short conclusion, Rain is a flexible offensive archetype, and it can be really fun to play. It has a lot of good matchups in modern ADV, and it would be really cool to see players experimenting with this type of playstyle, which is the main reason behind this thread :)
I would like to finish this thread by giving some shoutouts to important people to me in the Pokémon community and linking some amazing replays of Rain being used in tournaments/ladder games.
Replays:
https://replay.pokemonshowdown.com/smogtours-gen3ou-385578 - Lycomedes vs Dice - CALLOUS Cup 2
https://replay.pokemonshowdown.com/smogtours-gen3ou-422882 - Marcop vs VIL - SPL X
https://replay.pokemonshowdown.com/smogtours-gen3ou-390882 - thelinearcurve vs Lavos - CALLOUS Cup 2
https://replay.pokemonshowdown.com/smogtours-gen3ou-391344 - thelinearcurve vs Cowboy Dan - CALLOUS Cup 2
Feel free to sending me your Rain replays! :)
Shoutouts:
Dizno, Gacu, Lycomedes - First of all, thanks a lot for introducing me to ADV ladder and community. You guys are amazing persons and players who help a lot of people, revolutionize the meta every month and are always there for giving a tip, playing a game or just talk.
Hclat - Thanks a lot for all the advice and friendly games! You were the first person I approached when I was starting and you were always friendly. Very talented ADVer, I hope you get the deserved praise in the future. Also thanks for correcting my grammar in this thread :(
thelinearcurve - My favorite player. Thanks for inspiring me to write this thread and giving me the motivation to use Rain. You are always super friendly to me and I really appreciate that!
ADV Community Server members - You guys are awesome and the reason why ADV is the best old tier in the game. A lot of metagame discussion, advice, memes, etc. It’s super fun to be a part of this community and I’m really honored at the same time.
Thanks for reading; I really appreciate all the support/feedback :)!
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