Politoed (F) @Choice Specs
Ability: Drizzle
EVs: 252 SAtk/4 SDef/252 Spe
Timid Nature (+Spe, -Atk)
-Hydro Pump
-Ice Beam
-Focus Blast
-Hidden Power (Grass)
Standard Specs Politoed for tearing holes in anything not named Blissey or Chansey granted that you predict correctly. It helps Volcarona sweep by setting up the rain to make Hurricane 100% accurate, and weakening foes.
Volcarona (F) @Leftovers
Ability: Flame Body
EVs: 252 SAtk/4 SDef/252 Spe)
Modest Nature (+SAtk, -Atk)
-Quiver Dance
-Bug Buzz
-Fire Blast
-Hurricane
The star of the show. Despite being a fire type, its great on rain teams because it has a 4x resistance to grass, a weakness of most water types, and it gets 100% accurate Hurricane in rain. Combine that with Quiver Dance, arguably the best stat-upping move in the game, and Volcarona's excellent Special Attack and usable speed, and now it is a potent threat. Much of the team is focused around helping Volcarona sweep.
Ferrothorn (F) @Leftovers
Ability: Iron Barbs
EVs: 252 HP/88 Def/168 SDef
Impish Nature (+ Def, -SAtk)
-Spikes
-Protect
-Thunder Wave
-Power Whip
Ferrothorn is extremely useful. Rain weakens its biggest weakness, fire, and in turn Ferrothorn resists and powered up water type moves the opponent may have. It helps Volcarona sweep by setting up Spikes to make enemy Pokemon easier to KO, and it can use Thunder Wave to make sure Volcarona outspeeds its enemies. Protect lets it scout and gain more Leftovers recover, and Power Whip prevents it from being complete Taunt fodder.
Starmie @Life Orb
Ability: Natural Cure
EVs: 252 SAtk/4 SDef/252 Spe
Timid Nature (+Spe, -Atk)
-Surf
-Thunder
-Ice Beam
-Rapid Spin
Starmie is a great Pokemon. With Natural Cure, it can sponge status conditions, and with double STAB Surf (in the rain) as well as 100% accurate Thunder and Ice Beam to round out the coverage, it can really hurt teams late game. It helps Volcarona sweep by Rapid Spinning away Spikes and Stealth Rock, which otherwise greatly hinder Volcarona's ability to switch in.
Terrakion @Choice Scarf
Ability: Justified
EVs: 252 Atk/4 SDef/252 Spe
Adamant Nature (+Atk, -SAtk)
-Stone Edge
-Close Combat
-Earthquake
-Aerial Ace
Terrakion may seem like an odd choice on a rain team due to its grass and water weaknesses, but it is very useful. It not only checks opposing Volcarona with ease, but it also helps Volcarona sweep in many ways. Terrakion can seriously dent or OHKO outright all of Volcarona's counters (namely Blissey, Chansey, Heatran, Dragonite, and Gyarados). It can also attract grass type attacks, which Volcarona can switch in on easily,
Jirachi @Leftovers
Ability: Serene Grace
EVs: 252 HP /224 SpD /32 Spe
Careful Nature (+SDef, -SAtk
-Wish
-Stealth Rock
-Body Slam
-Iron head
Jirachi is another great Pokemon for rain teams, with one of its two weaknesses being negated by rain, and resisting grass attacks. It helps Volcarona sweep by using Stealth Rock to wear down Volcarona's checks and counters, using Body Slam to paralyze foes so Volcarona can set up and outspeed them, and it can use Wish and then switch to Volcarona so Volcarona gains 202 HP back, greatly increasing its longevity.