Ninetales @ Leftovers
Ability: Drought
Nature: Timid
EVs: 252 HP, 128 Speed, 128 Def
- Protect
- Roar
- Will-O-Wisp/ Solar beam
- Flamethrower
I would rather go defensive on Ninetales. Your entire team is based around sun, so you should preserve your weather lead for as long as possible. OU is ridden with weather teams, and should you face a Sand/ Rain team, you will be at a massive disadvantage if Ninetales faints early on.
Darmanitan @ Life Orb
Ability: Sheer Force
nature: Adamant
EVs: 252 Atk, 252 Speed, 4 Def
Flare Blitz
Rock Slide
Zen Headbutt
U-Turn
While I understand why you chose Fire Punch over Flare Blitz for lack of recoil, you need to sweep as fast as possible. Fire Punch will not OHKO Pokemon that take neutral damage from fire. Darmanitan is really frail, so you need to maximise its use on the battlefield by giving it the most powerful move. Zen Headbutt and Rock Slide will be the ones you can use w/o Life Orb recoil; Rock Slide over Earthquake because no Life Orb damage and it will hit flying types.
Arcanine @ Life Orb
Ability: Flash Fire
Nature: Jolly
EVs: 252 Sp. Atk, 252 Speed, 4 HP
* Solar Beam
* Flamethower
* Extremespeed
* Morning Sun
While your Arcanine is a special sweeper, I would give it Extremespeed against Pokemon that might outspeed you in Sun, i.e. those with Chlorophyll. It'll be able to get rid of any laggers with low HP by using a priority move.
Roserade @ Life Orb
Ability: Natural Cure
Nature: Modest
EVs: 252 Sp. Atk, 252 Speed, 4 HP
* Giga Drain
* Sludge Bomb
* Leaf Storm
* HP Fire/ Sleep Powder
HP Fire > Weather Ball, just in case the weather changes. I don't like one use items like gems in most cases, so give your Roserade a Life Orb as well. Recoil damage won't do too much with STAB Giga Drain.
Bibarel's not suited for OU. Also, Double Team is banned from all tiers other than Uber. For a water counter, I would suggest Tentacruel, which will double as a Rapid Spinner.
Tentacruel @ Black Sludge
Ability: Liquid Ooze/ Clear Body/ Rain Dish
EVs: 252 HP, 252 Def, 4 Speed
Nature: Timid/ Bold
* Rapid Spin
* Scald/ Ice Beam
* Knock Off
* Protect/ Toxic Spikes
Tentacruel's advantageous typing means it can take hits from many things, soak up Toxic Spikes, and Grass is now x1. It will also clear entry hazards for you, and if you want you can set up your own hazards as well.
All three of Tentacruel's abilities are viable in this situation. In the scenario you are facing a Rain team, Rain Dish will be beneficial. Liquid Ooze will prevent Pokemon from using Giga Drain on it. Clear Body will keep its stats from lowering. You should decide which ability will benefit Tentacruel the most.
Tropius is another NU Pokemon and is not suited to OU. You have no Chlorophyll user on this team, and Tropius' slow speed, mediocre defences and poor typing means it will easily get swept before Harvest can do its job. If you wanted to use something with Harvest, I would suggest Exeggutor. However, I think it would be better if you abused Drought to the maximum and used a Pokemon with Chlorophyll.
For a Chlorophyll sweeper, here are two choices:
Sawsbuck @ Life Orb
Ability: Chlorophyll
EVs: 252 Speed, 252 Atk, 4 HP
Nature: Adamant
* Horn Leech
* Swords Dance
* Nature Power/ Jump Kick
* Return
Venusaur @ Life Orb
Ability: Chlorophyll
EVs: 252 Speed, 252 Sp. Atk, 4 HP
Nature: Modest
* Growth
* Giga Drain
* Sludge Bomb
* HP Fire
Venusaur's typing will overlap with Roserade's, but then again Sawsbuck doesn't really have any added useful resistances apart from Ghost immunity. The choice between these two is whether you want another physical sweeper or a special sweeper. However, Venusaur can also be made into a Physical sweeper with Earthquake, Seed Bomb and Return, but it forfeits Giga Drain as a HP restoring method. Sawsbuck will have Horn Leech, and its Attack is superior in any case, so Sawsbuck is mainly a physical Pokemon.
This team is high on the offensive, so you should abuse your weather to the fullest with maxed out attacks/ special attacks, along with abilities like Chlorophyll. Your aim is to sweep as fast as possible and as effectively as possible, so there's no need for Blissey to act as Cleric because you want to be able to wipe out your opponent before they can do anything to you.