Ok so I lack time so I am going to go through this quickly.
Politoed
Honestly no weather inducer can afford to risk switching in on it so Rain Dance is useless. Perish song is nice but is only ever really needed for Pokemon that set up, which your team deals with pretty well. An offensive specs set will really suit your team.
Cloyster
Rapid spin and Focus sash on the same set spells fail. Razor Shell is much better and not only gets stab but gets a rain boost. Though your team lacks rocks and a Pokemon with solid def in both areas. So I suggest Jirachi.
Toxicroak
You have the right idea but you would get much more out of it if your ran swords dance. Also Protect is useless here really.
Azumarill
Perfect though Superpower is needed to hit Ferro.
Thundarus-T
First off he needs a new sprite :P Second running a scarf on it is a solid option as currently your whole team fails against opposing Thunderus-T so a speedy check is needed. Your team also appreciates the momentum gained by a scarf.
Salamence
Your team isn't being held back by Salamence but your team really rely's on setting up and needs some better checks to Pokemon like Rotom-W. Landorus-T would sound a weird option at first but it has Gravity that allows it to kill Rotom on the switch in. It also hit 10x harder than Salamence at no boosts. Volt turn is also nice.
Sorry for very quick rate. Here is the edited team.
Politoed @ Choice Specs
Trait: Drizzle
EVs: 40 Spd / 252 SAtk / 216 HP
Modest Nature
- Hydro Pump
- Hidden Power [Grass]
- Ice Beam
- Focus Blast
Jirachi @ Leftovers
Trait: Serene Grace
EVs: 32 Spd / 252 HP / 224 SDef
Careful Nature
- Body Slam
- Iron Head
- Wish
- Stealth Rock
Toxicroak @ Life Orb
Trait: Dry Skin
EVs: 232 Spd / 24 HP / 252 Atk
Jolly Nature
- Swords Dance
- Ice Punch
- Sucker Punch
- Drain Punch
Azumarill @ Choice Band
Trait: Huge Power
EVs: 252 Atk / 248 HP / 8 Def
- Waterfall
- Aqua Jet
- Ice Punch
- Superpower
Thundurus-Therian (M) @ Choice Scarf
Trait: Volt Absorb
EVs: 252 SAtk / 252 Spd / 4 HP
Modest Nature
- Thunder
- Volt Switch
- Hidden Power [Ice]
- Focus Blast
Landorus-Therian (M) @ Life Orb
Trait: Intimidate
EVs: 4 HP / 252 Spd / 252 Atk
Naive Nature
IVs: 30 Atk / 30 Def
- Earthquake
- U-turn
- Gravity
- Stone Edge
