http://www.smogon.com/dp/pokemon/
Anyways, the set was already written here by ARandomBoy, but I changed the set a bit and rewrote the comments. The rest of it I wrote.
QC APPROVALS: 2/2 [HEYSUP][KD24]
GP APPROVALS: 2/2 [BOJANGLES] [GREAT SAGE]
Status: Ready for Upload!
[OVERVIEW]
<p>Goldeen is often overlooked because of its all-around average base stats. However, when Rain Dance is set up, Goldeen can become a very strong sweeper. With Megahorn exclusive to it among Rain Dance sweepers, Goldeen can kill Snover, one of Rain Dance's biggest nemeses. With its decent bulk, Goldeen can also set up Rain Dance for itself and its teammates. For these reasons, Goldeen is a valuable addition to any Rain Dance team.</p>
[SET]
name: Swift Swim Sweeper
move 1: Waterfall
move 2: Return
move 3: Megahorn
move 4: Rain Dance / Psybeam
item: Life Orb / Oran Berry
nature: Adamant
ability: Swift Swim
evs: 220 Atk / 36 Def / 252 Spe
ivs: 0 HP
[SET COMMENTS]
<p>This set abuses Goldeen's ability, Swift Swim, as well as its advantages over other Rain Dance sweepers in Megahorn, Psybeam, and its greater bulk.</p>
<p>Rain-boosted Waterfall hits like a truck, being able to OHKO common threats in Little Cup, such as Gligar and Gastly. Return provides perfect neutral coverage with Waterfall while hitting Water-type resisters, such as Mantyke. Megahorn is the crux of the set, OHKOing Snover, who normally stops Rain Dance teams cold with Snow Warning. It also OHKOes Carvanha and has a chance to OHKO Wynaut after Stealth Rock damage. Rain Dance allows Goldeen to set itself andits teammates up. Psybeam OHKOes Croagunk, another bane of Rain Dance teams. Be warned, however, that Croagunk's vast range of priority attacks will either cripple or outright KO Goldeen.</p>
[ADDITIONAL COMMENTS]
<p>Goldeen's item choice matters a lot to this set. With Life Orb, Goldeen guarantees the OHKO on Wynaut and has a chance to OHKO Munchlax in rain with Waterfall. However, with Oran Berry, Goldeen has a much easier time setting up Rain Dance. It also takes on Croagunk much better due to being able to heal off Croagunk's priority attacks.</p>
<p>The EVs give Goldeen maximum Attack and Speed. The rest goes into Defense for an extra point. A 0 HP IV is to reach 19 HP, meaning Goldeen will only take one HP of damage from Life Orb. If Oran Berry is used, the HP IV should be left at 31.</p>
[Team Options]
<p>It is best for Goldeen to be on a Rain Dance team because Goldeen appreciates having Rain Dance set up before it comes in. The main candidates for Rain Dance setup are Voltorb and Bronzor. Voltorb usually occupies the lead spot, as it can throw off a fast Rain Dance and then use Explosion or hit hard with a STAB Thunder. Bronzor is one of Little Cup's bulkiest Pokemon and it sets up Rain Dance with incredible reliability. Having Rain Dance set up beforehand also means that Goldeen can use Psybeam to hit Croagunk.</p>
<p>If Goldeen is setting up Rain Dance itself, it is helpful to support it with Reflect and Light Screen. These screens will increase Goldeen's above-average bulk to levels where it can set up and sweep easily. Bronzor, Abra, and even Slugma can set up screens behindwhich Goldeen can sweep.</p>
<p>Goldeen has a huge weakness to Chinchou, only being able to hit it with Return while it OHKOes back with Thunderbolt. Therefore, a Chinchou counter is recommended. Diglett is one of the best Chinchou counters, especially on a rain team. Diglett can switch into Thunderbolt, trap, and OHKO Chinchou with Earthquake. Munchlax can hit Chinchou hard with Return or Earthquake while taking minimal damage from Chinchou's attacks. Munchlax has the added advantage of taking out Mantyke, who can survive Goldeen's attacks.</p>
[Optional Changes]
<p>Goldeen has little utility outside of sweeping in rain. A Scarf set with Megahorn could be usable, reaching 17 Attack and 24 Speed. However, it lacks power without a rain-boosted Waterfall; Buizel generally does a better job. Agility could be used over Rain Dance, but Waterfall again lacks the power required to sweep. Finally, Aqua Tail could be used for the slight power boost over Waterfall, but the 90% accuracy could occasionally be a letdown.</p>
[Counters]
<p>Goldeen needs rain to sweep effectively. Therefore, with rain removed, Goldeen becomes much less of a threat. However, the two weather changers in Little Cup, Snover and Hippopotas, are threatened by Megahorn and Waterfall, respectively. With this taken into account, Snover and Hippopotas are not effective counters to Goldeen.</p>
<p>Waterfall cannot OHKO Munchlax unless it switches into Stealth Rock and Goldeen rolls maximum damage with Life Orb under rain. Munchlax can then do massive damage back with Return. The same thing applies to Chinchou and Mantyke, who are only hit by Return and can OHKO or 2HKO back with Thunderbolt and Hidden Power, respectively. Finally, Croagunk can take down Goldeen with priority attacks. It must watch out for Psybeam, however.</p>
