So as Marzipan said you are not abusing the rain. Now that's not so much of a problem with Politoed having a nice niche in Ou outside of Drizzle unlike Ninetales, but you are not abusing rain in places where you should be or could be. Because of how the teams built I actually will not be able to fix this problem so much but the suggestions made hopefully will help balance out the team.
The synergy of the team is nice though changes will have to be made because though the synergy is nice three Pokemon are dead weight on the team and offer nothing to your offense and even defense for the most part. This is partly to do with how they preform and also the sets they are running which are honestly not so great.
Sets will be below
So currently your team is just sat on by a simple Gastrodon and Heatran core. So I would run Toxic over Hypnosis. This way you can wear Gastrodon and other Pokemon that otherwise just wall you outside of steel typing. You still have Perish Song to prevent Latias and Reuncluss setting up. Another change you could make is Protect over Ice Beam. Protect helps scout what the opponent is going to do and really helps when you know the opponent is choiced. It also helps stall with Toxic and Leftovers. Ice beam is not needed on the set seeing as your running defensive. If your worried about Dragonite Toxic is a nice move to cripple it along with Scald and its 100% burn rate...
Toxic > Hypnosis
Protect > Ice Beam
I'm not going to lie I really don't like this set. Sure Doom Desire has a nice 140 BP but considering it takes 3 turns to hit you may as well just run Iron Head which really abuses it's ability. Doom Desire is the only real thing setting Jirachi back here and I highly suggest you loose it. There is not one single argument for why Doom Desire is better.
Your team also lacks a form of entry hazards which is a must in Ou to stop threats like Volcarona constantly switching in. Now your two main candidates here are Jirachi and Celebi both of which can very successfully run a nice special defensive set. Now because Celebi is such a sturdy wall in the current meta I am not going to run rocks on it as it would really appreciate the free slot. Instead I will be replacing Gliscor and Gastrodon for Landorus-T. Now because your team is so bulky I am starting to wonder if your missing out on some much needed offense.
For this I recommend ScarfRachi who is a solid late game sweeper and revenge killer.
ScarfRachi > Current Rachi
Honestly what is it doing here? Its a one man band offering nothing to the team. The set isn't even taking full advantage of its potential. You already have one solid switch in to Water attacks in Celebi who also isn't broken my a Politoeds clever prediction going for the common HP Grass. It offers no support in Toxic and is pretty open to it itself. You will always be out stalled by any Toxic staller and Ferrothorn.
For this reason I highly suggest running Landorus-T. It can set up rocks and has decent synergy with the team. A Defensive pivot set would be my recommendation.
DP Landorus-T > Gastrodon
Gliscor is your second Pokemon who is not so welcome on the team. The set will be not hitting very hard even at +2. Your not abusing its solid defense, apart from the fact its loosing its popularity in Ou due to new threats.
The lack of Ice coverage is noticeable so Mamoswine seems to be a solid replacement. It deals with Landorus and Thundurus-T who really trolls with the team.
Mamoswine > Gliscor
Celebi is the final member who is dead weight on the team. It has a really good solid place on the team but your not abusing that. Instead your using a NP Baton Pass set which isn't great with the amount of speedy U-turners there are. Instead I highly recommend a Special Defensive set. If used correctly it can wall entire teams. It is by far one of the best walls in the tier at the moment and your team really would appreciate its ability to walls Specs Keldeo as nothing else really fancies taking a Specs Hydro Pump. This change would be one of the most beneficial to the team.
Special Defensive > NP BP
This set fits nicely on the team. Though personally I prefer a Choice Specs set to abuse Volt Turn and hit everything for kos with Thunder. Though your current set is fine. Your spread whereas is maybe not so great. Run a more offensive spread with enough speed to at least out speed Scizor and SAtk investment to actually land Kos.
More offensive ev spread.
Importable considering all changes are taken into account/Sets
Politoed @ Leftovers
EVs: 252 HP / 252 Def / 4 SDef
- Perish Song
Jirachi @ Choice Scarf
Trait: Serene Grace
EVs: 252 Atk / 252 Spd / 4 SDef
- Iron Head
- Ice Punch
Landorus-Therian (M) @ Leftovers
EVs: 200 HP / 64 Atk / 244 Def
- Stealth Rock
- Stone Edge/Superpower/Hidden Power Ice
Mamoswine @ Choice Band
Trait: Thick Fat
EVs: 252 Atk / 252 Spd / 4 Def
- Ice Shard
- Stone Edge
Celebi @ Leftovers
Trait: Natural Cure
EVs: 20 Spd / 252 HP / 236 SDef
- Baton Pass
- Giga Drain
- Thunder Wave
Magnezone @ Leftovers
Trait: Magnet Pull
EVs: 92 Spd / 252 SAtk / 164 HP
- Hidden Power [Fire]
- Flash Cannon
This actually looks like a very solid team. Any sets your confused about just let me know.
But for now Magnezones speed evs lets it out speed scizor and standard Poli as you don't want to take a Hydro Pump, even with the resistance it does a lot. Celebi has Baton Pass as an alternative to U-turn so it doesn't get trapped by Scizors and Tyranitars pursuit. It also plays very nicely when the opponent switches as instead of switching yourself you can choose your Pokemon knowing what your up against. Mamo needs max speed so Mach Punch Breloom doesn't screw it over.
Overall I really like the new team :3 good job on the first team and the nice win to loss ratio. I highly recommend you take some of these if not all of these suggestions into account.