I like the idea. To put Castform in a competitive domain means that you are either a fan or a stupid person, because Castform is piss weak. However, in NU -its home, you can try to make it work. However, as Sempi stated, Castform, in three words: piece of nullity.
The best mates for Castform in NU with the Sun up are Volbeat, Liepard, Murkrow, Camerupt, Torkoal and Drifblim.
Volbeat sets the sun with Prankster, and its role is to make Castform strong by doing a Tail Glow then a Baton Pass to Castform, who will start spamming Weather Ball for some turns. You can also dispatch your foe with Thunder Wave, which is vital for Castform, for it is as slow as Samurott.
Liepard also works like Volbeat: Prankster Sunny Day, but Liepard has also the gimmick Assist if you are lucky. Liepard is frailer than Volbeat or Murkrow, but it has Encore and Taunt, two deadly moves in tandem.
Murkrow needs the Eviolite to work and is very similar to Liepard and Volbeat. However, it has got support moves that the others lack, like Roost, Tailwind or even FeatherDance.
Camerupt can be made defensive or offensive at will. It needs Sunny Day, then three moves of your choice. The list is appealing: Lava Plume, Yawn, Will-O-Wisp, Earth Power and Solid Rock has ability makes it a bit more resistant to Water than other Pokémon like it. Ah, and Camerupt, while being cute, can also use Stealth Rock, which will simply help Castform sweeping.
Torkoal can be an amazing supporter, but it destroys the momentum of a quick-based team. It can Rapid Spin, which is well coveted, and has access to boosted Lava Plume. I'd not chose it, but it is up to you.
Drifblim can set up Sunny Day like another Pokémon, but it has, like Castform, access to Weather Ball. It has got Will-O-Wisp and Acrobatics as well. If you chose the Fire Gem, Unburden activates. If not, then forget the speed in favor of a long Sunny Day when Heat Rock equipped.
You will need abusers then.
Sawsbuck is one of the most dangerous Pokémon to take out of a team once sun is up. It reaches 634 Speed with Chlorophyll and Jolly under the sun. Life Orb equipped, Swords Dance, +2 Double Edge will kill pretty much the whole tier. It has Horn Leech to restore Double Edge and Life Orb's unwanted recoil, and Nature Power bypasses Sucker Punch and acts like Earthquake. You want more ?
Exeggutor's Chlorophyll makes it sorta quick, and Harvest recycles its berries all the time. STAB SolarBeam is deadly from 125 Special Attack, and Psychic and Hidden Power Fire are good as well.
Victreebel is also a monster, due to Chlorophyll and Growth. With Growth under the Sun, Victreebel is unstoppable. It has access to Weather Ball, turning into a stronger Hidden Power Fire and SolarBeam and Sludge Bomb are deadly.
Cherrim boosts your team's special defense and attack by 50 % with Flower Gift, which is incredible. It can then Growth once, and spam Seed Bomb -its best STAB, Nature Power -dispatching Sucker Punch and turning into Earthquake, and Return to catastrophic holes in your foe's team.
You then need Fire-type.
Charizard with Solar Power in Sun is a powerhouse. I mean wut. Fire Blast, SolarBeam are its best asset. Choice Specs slapped, it 2HKOes the whole NU tier, but Flareon is troublesome; you will either need 2HKO with Air Slash or closer try with Hidden Power Rock or Return.
Rapidash with its blazing sweeping stats is appealing. It has Megahorn, Flare Blitz, Wild Charge, Drill Run, Poison Jab, Low Kick as its physical asset, and SolarBeam can be used to beat Quagsire and Golem. Flash Fire makes it stronger against foe Fire-types and Drill Run dispatches them, while carrying Flash Fire Boost.
Combusken: Slap an Eviolite, Speed Boost, make it mixed, Swords Dance, Flare Blitz, Focus Blast and Protect and you have what resembles the Uber Blaziken, in weaker modo and NU. Mantine and Shelgon can't stall.
In short, Castform is hard to use. If your team is not successful, blame me not. Castform is too weak.