Okay Leboss, here I am! I'm going to try to give this team some improvements.
First off, Politoed looks like a Scarftoed with the EV spread of a Specstoed.
You might want to change that up to a Scarftoed.
Politoed @ Choice Scarf
EVs: 4 HP / 252 SAtk / 252 Spd
Modest Nature (+SAtk, -Atk)
- Hydro Pump
- Ice Beam
- Perish Song
I don't really see what pokes Timid is important for except for Tornadus-T which is now Uber. This set can't really touch CM Jirachi or CM Reuniclus or CM Keldeo after a few boosts, so Perish Song is here to force them out. It's also generally a good move against sub pokes and the choice scarf should help you outspeed them.
For Latios, since you have no Wish support or recovery on Latios, Recover can be considered instead of probably Tbolt. Although Tbolt might have some use against pokes like Politoed, other stuff it's useful for like Skarmory and Keldeo are 2HKOed for Skarmory and a Psyshock with STAB and Life orb should take Keldeo out.
Other than that, Latios can be a nice member to rain teams, helping to counter Keldeo which seems like a threat to your team.
For Thundurus-T I'd suggest Agility. Agility could do well as a late game sweeper, after your other attackers have weakened the opponents team.
Nasty Plot with a Salac Berry doesn't actually seem like it could sweep very often, with pokes like Scarf Keldeo and Terrakion in the meta plus Thundurus-T isn't the bulkiest Poke in the world, so it would have to take an attack and perhaps even rocks damage.
Thundurus-T @ Life Orb
Trait: Volt Absorb
EVs: 4 Def / 252 SAtk / 252 Spd
Timid Nature (+Spd, -Atk)
- Hidden Power Ice
- Grass Knot/Focus Blast
Since you won't always have the opportunity to use agility, Timid max speed is nice so you can still fire off attacks even when unboosted. Life Orb does give Thundurus-T an impressive kick though.
Gastrodon and Quagsire seems like pests to this team, so I put Grass Knot there which also hits incoming Mamoswines wanting to pich you off with Ice Shard, but feel free to use Focus Blast if you don't feel the need for it or you just prefer Focus Blast.
If you want even more power, you can go with an EV spread of 70 Def / 252 SAtk / 188 Spd with a Modest nature which outspeeds timid stoutland in the sand and jolly Venusaur in sun.
This spread also helps Thundurus-T take attacks like Mach Punch from Breloom, Bullet Punch from Scizor, Gyro Ball from Ferrothorn, and Iron Head from Jirachi even better.
On Forretress I suggest some more SpDef EVs or a Specially defensive spread.
Forretress @ Leftovers
EVs: 252 HP / 176 Def / 80 SDef
Relaxed Nature (+Def, -Spd)
- Stealth Rock
- Rapid Spin
- Gyro Ball
- Volt Switch
This should also help you takes Scalds and Ice Beams better.
You could also go 252 HP / 4 Def / 252 SDef with Sassy and the same moveset.
Gyro Ball helps against Gengar spinblockers and Terrakion as well.
Conkeldurr I suggest putting a choice band on.
Conkeldurr @ Choice Band
Trait: Iron Fist
EVs: 176 HP / 252 Atk / 80 SDef
Adamant Nature (+Atk, -SAtk)
- Hammer Arm/Drain Punch
- Mach Punch
- Ice Punch/ThunderPunch
Conkeldurr is a threat, and this set helps you widdle down the opponents team even more so Thundurus-T can come in and late game sweep.
Hammer Arm is the main STAB, and is boosted by Iron Fist.
You could also go with Drain Punch if you want healing, and Drain Punch is boosted by Iron Fist as well so it does also nice amounts of damage.
Mach Punch is also boosted by Iron Fist and is nice priority and also being STAB.
Payback hits incoming Ghosts. Ice Punch hits Gliscor, Landorus-T, Dnite, Salamence and other stuff that's weak to Ice in the OU meta. ThunderPunch hits Jellicent and Gyarados and Skarmory plus Tentacruel although a Hammer Arm should do a lot to Skarm if you choose to run that.
For the last Pokemon I think you need a scarfer, and I've been looking for a Poke that isn't weak to water or ice and is a physical attacker. I'd suggest Gyarados.
Gyarados @ Choice Scarf
EVs: 252 Atk / 4 Def / 252 Spd
Jolly Nature (+Spd, -SAtk)
- Ice Fang
- Stone Edge
Gyarados is the scarfer of the team, and also helps you take physical hits with Intimidate.
You could also go with Moxie, but I thought Intimidate would be nice for the reason stated above. Waterfall is boosed by STAB and Rain, Earthquake hits Tentacruel Supere effectively and Jellicent neutrally and other stuff as well.
Ice Fang revenges Dragons like Dnite, and Stone Edge gets EdgeQuake along with Earthquake.
I think that's it, hopefully this helped!