Ok a few suggestions.

First off is to drop Kingdra. Yeah I know, he's the coolest looking Dragon ever. Clair may have utterly devastated teams with it but it sucks *** in OU right now. It gets walled to death by Sp Def Celebi, Ferrothorn, Jellicent, Blissey, Vaporeon etc. It has some power but your team doesn't really even handle its counters too well. What your team does do very well however is work in Rain. Running Gyarados without rain is almost like trying to sweep with Gyarados when it is constantly intimidated. Try Politoed > Kingdra.

Politoed @ Leftovers
Trait: Drizzle
EVs: 244 Def / 252 HP / 12 Spd
Bold Nature
- Scald
- Toxic
- Perish Song
- Protect

Defensive Politoed gives you something to do against a +1 Gyarados, besides lose. It also brings Rain and can help lure in and burn Ferrothorn / Toxic Rotom W. With Ferrothorn burned and Rotom W crippled or dead very little can stand in your Gyarados's way.

Mamoswine is mostly fine. However try running Focus Sash with Stealth Rock > Icicle Crash. This way you can run Spikes on Ferrothorn. Also on Ferrothorn try Thunder Wave. Twave / Spikes + 2 filler (pick 2 of Leech Seed / Gyro Ball / Power Whip / Protect).

In testing I found Breloom to be utterly useless. It never really managed to sweep so more often than not I just hit something with Spore and double switched out as they brought in their Dragonite or whatever. If you are running Politoed > Kingdra (I HIGHLY suggest you do) try Specially Defensive Calm Mind Jirachi.

Jirachi @ Leftovers
Trait: Serene Grace
EVs: 176 Spd / 252 HP / 80 SDef
Calm Nature
- Substitute
- Calm Mind
- Thunder
- Water Pulse

HP and Sp Def EVs gives you enough durability to prevent Reuniclus and Tentacruel from ever breaking your Subs (Reuniclus has like a 10% chance to but oh well). Speed outruns Adamant Lucario. Water Pulse has 90 BP in the rain and rips through annoying defensive Landorus and Gliscor while breaking Heatran and sorta hurting offensive Tyranitar.

On Gyarados run Bounce > Stone Edge. Bounce OHKOs Salamence, Gyarados, and Dragonite after rocks while Volcarona obviously can't take a Waterfall. Rotom W takes more damage from Stone Edge but Rain boosted Waterfalls have 90 BP vs Stone Edges 100 and you can OHKO Scarf Rotom with +2 Rain Waterfall. Bounce lets you break through Celebi and Ferrothorn, even spreading Paralysis and stalling for Leftovers recovery. It even has better accuracy and power. It is absolutely the better option.

Scarf Latios is ok, but try Scarf Latias instead. Though it has slightly worse offenses (though they are by no means bad) it has the special defense to check things like Politoed / Thundurus / Venusaur etc more easily. Not only that but it gets Healing Wish which is amazing for your team. Healing Wish is a great option for Gyarados and Jirachi. If a Sub sweeper is forced out before it wins the game it generally doesn't have many more chances to sweep. However Healing Wish will let you bring Gyarados, Jirachi, or even Politoed (keep Rain up) back from the brink of death. Healing Wish also heals before Spikes / SR / Sandstorm damage so a 1% Gyarados will end up at decent HP even if rocks are down. Healing Wish also brings your Pokemon back at the end of the turn so you don't even need to switch in. Healing Wish can bring back a 1% Mamoswine.

This is the importable I ended up with. I know you don't want to lose Kingdra but dropping Kingdra could be the difference between having an average team and having a very solid, very usable one.

    Spoiler:- team: