(Zebraiken approved that I could post this despite having a few other analyses still in c&c because the GP team is taking a while to GP Musharna. (No offense to them; I realize that they're busy.)
Best Mantine set.
[SET]
name: Rain Dance
move 1: Rain Dance
move 2: Hydro Pump
move 3: Air Slash
move 4: Ice Beam
item: Life Orb / Damp Rock
ability: Swift Swim
nature: Modest
evs: 64 HP / 252 SpA / 192 Spe
[SET COMMENTS]
<p>Rain Dance Mantine is a fantastic sweeper and team supporter. At first, it might seem as if it is outclassed by other rain sweepers such as Seismitoad and Ludicolo because of its lower Special Attack. However, Mantine has other desirable traits. It has a very easy time setting up Rain Dance due to its amazingly high Special Defense, and Mantine's Flying typing actually has its uses. While the additional Flying typing makes Mantine weak to Stealth Rock, it also gives Mantine more setup opportunities, as it gives Mantine a resistance to Ground-, Fighting-, and Bug-type attacks. Mantine also gets STAB Air Slash, allowing it to break through Ludicolo and Shedinja, who are otherwise gigantic threats to rain teams that lack something such as Swift Swim Armaldo. Ice Beam is useful for hitting the Dragon-types that resist Hydro Pump, especially Altaria.</p>
[ADDITIONAL COMMENTS]
<p>With 192 Speed EVs, Mantine can outspeed Choice Scarf Rotom-S in the rain. Less Speed EVs can be used to make Mantine more of a bulky supporter rather than a sweeper, but if Mantine is used purely for standalone sweeping, it should have enough Speed to outspeed Choice Scarf Rotom-S in the rain. Life Orb is the preferred item to give Mantine the extra power that it needs. Despite this, only having four turns to sweep isn't exactly that much; Damp Rock is a good option over Life Orb, but the power drop will be noticeable. Leftovers can be used over Life Orb or Damp Rock for some sort of recovery, which is useful when Mantine is trying to check certain threats such as Charizard, but Leftovers doesn't aid Mantine at all when it comes to sweeping.</p>
<p>Using multiple rain sweepers with Mantine is a good strategy. Mantine can act as the bulky supporter that can also sweep in a pinch while Seismitoad, Ludicolo, or some other Swift Swim user can sweep. Seismitoad synergizes the best with Mantine, as it resists Rock- and Electric-type attacks for Mantine, while Mantine can easily beat the Grass-types that threaten Seismitoad. In addition, Seismitoad can set up Stealth Rock. Fighting-types such as Gurdurr and Sawk can beat many Pokemon that threaten rain sweepers, such as Lickilicky, Audino, and Regice, making them good partners as well.</p>

Best Mantine set.
[SET]
name: Rain Dance
move 1: Rain Dance
move 2: Hydro Pump
move 3: Air Slash
move 4: Ice Beam
item: Life Orb / Damp Rock
ability: Swift Swim
nature: Modest
evs: 64 HP / 252 SpA / 192 Spe
[SET COMMENTS]
<p>Rain Dance Mantine is a fantastic sweeper and team supporter. At first, it might seem as if it is outclassed by other rain sweepers such as Seismitoad and Ludicolo because of its lower Special Attack. However, Mantine has other desirable traits. It has a very easy time setting up Rain Dance due to its amazingly high Special Defense, and Mantine's Flying typing actually has its uses. While the additional Flying typing makes Mantine weak to Stealth Rock, it also gives Mantine more setup opportunities, as it gives Mantine a resistance to Ground-, Fighting-, and Bug-type attacks. Mantine also gets STAB Air Slash, allowing it to break through Ludicolo and Shedinja, who are otherwise gigantic threats to rain teams that lack something such as Swift Swim Armaldo. Ice Beam is useful for hitting the Dragon-types that resist Hydro Pump, especially Altaria.</p>
[ADDITIONAL COMMENTS]
<p>With 192 Speed EVs, Mantine can outspeed Choice Scarf Rotom-S in the rain. Less Speed EVs can be used to make Mantine more of a bulky supporter rather than a sweeper, but if Mantine is used purely for standalone sweeping, it should have enough Speed to outspeed Choice Scarf Rotom-S in the rain. Life Orb is the preferred item to give Mantine the extra power that it needs. Despite this, only having four turns to sweep isn't exactly that much; Damp Rock is a good option over Life Orb, but the power drop will be noticeable. Leftovers can be used over Life Orb or Damp Rock for some sort of recovery, which is useful when Mantine is trying to check certain threats such as Charizard, but Leftovers doesn't aid Mantine at all when it comes to sweeping.</p>
<p>Using multiple rain sweepers with Mantine is a good strategy. Mantine can act as the bulky supporter that can also sweep in a pinch while Seismitoad, Ludicolo, or some other Swift Swim user can sweep. Seismitoad synergizes the best with Mantine, as it resists Rock- and Electric-type attacks for Mantine, while Mantine can easily beat the Grass-types that threaten Seismitoad. In addition, Seismitoad can set up Stealth Rock. Fighting-types such as Gurdurr and Sawk can beat many Pokemon that threaten rain sweepers, such as Lickilicky, Audino, and Regice, making them good partners as well.</p>