I realize my answer is a bit late, but I find Castform to be a fun challenge and I wish to contribute for those who happen to stumble upon this Question.
Quick Note
This may seem counter intuitive, but Castform isnt well suited for setting up weather (or much else) and it works best on teams that specialize in ONE weather condition. Due to having poor base stats, it requires every turn it can get to attack. Having one or more teammates able to set up weather for Castform gives it the best chances for success.
RAIN
As far as which weather is best for Castform, I find Rain is most accommodating. In addition to boosting Weather Ball, it allows Castform to make use of Hurricane and Thunder which, provide excellent coverage against Grass and Water types, as well as strong alternatives to Weather Ball should something swap in and resist it.
Possible Teammates
PU
Raichu has a great support movepool and, with Lightning Rod, it helps shelter Castform and its team from incoming electric attacks. Nuzzle cripples faster Pokemon, and pairing Rain Dance with Thunder provides useful setup for your team as well as a powerful hit against anything that doesn't resist it.
Pelliper is a fantastic defensive Pokemon able to make use of the ability, Rain Dish. Using Scald, which not only is boosted under Rain, but has a chance to burn, only helps its ability to wall physical threats. Another option for it would be a Uturn/Tailwind set, which would pair nicely with a Volt Switch Raichu or Pachirisu.
Floatzel, Huntail and Basculin are great physical attackers. Floatzel has high speed and its ability Water Veil gaurds against burns. Huntail also makes use of Water Veil and access to Shell Smash makes it a very formidable Pokemon. Although Basculine does have a lot of power under Rain and with Adaptability, this immense power is limited to water type moves.
Whiscash has an outstanding Hp stat and can jump in on Electric attacks. Rain Dance and Rest gives it great recovery when paired with Hydration, and Scald fits well within this set. Having Toxic puts a timer on walls and using Earth Power rather than Scald provides a way to counter Steel types.
NU
Ludicolo Makes great use out of its Grass/Water typing, which removes Water's weaknesses to both Electric and Grass. This Pokemon provides great support with Rain Dance and has strong special defensive capabilities. Access to Leech Seed and the ability Rain Dish allows it the option to use a damp rock rather than leftovers which helps keep rain up longer.
Kabutops or Poliwrath have great physical capabilities that pair nicely with Swift Swim. The deference between them being Poliwrath has better coverage, where as Kabutops has higher attack and access to Aqua Jet which is a strong, STAB priority attack under Rain.
SUN
The next best weather for Castform is Sun.
Though not as supporting as Rain, Sun is generally a more offensively empowering weather condition for your team. Castform becomes a fire type, and adding Solar beam to its moveset provides coverage to all its type weaknesses. Although there are not as many options for strong moves, Ice beam is a still good option as it helps take out threatening Dragon types that Weather Ball and Solar Beam will be useless against.
Possible Teammates
PU
Tropius has nothing but useful abilities under the Sun and its immunity to ground makes it a great swap in for saving Castform.
With Chlorophyll activated, Swords Dance can be a great way help it pick off Pokemon that wall Castform's Special Attacks. Solar Power provides an Additional special attacker for your team. Harvest can contribute to Tropius as a Defensive or Special Defensive Pokemon. There are alot of options so I will leave this Pokemon's set details up to the Trainer's discretion.
Leafeon is another fantastic choice for a Sun team. High attack power and Chlorophyll makes it a lethal physical attacker. Leaf Blade benefits from STAB and comes with a high critical hit chance as an added perk. Knock off generally does well on any Pokemon and X Scissor takes out other Grass Types. Leafeon has a decent speed stat on its own, so having the ability Leaf Guard to defend against burns is another viable option.
Leavanny is great at setting up Sun and Sticky Web. If outsped, This Pokemon can set up Sun the first turn, survive with a Focus Sash, and guarantee the sticky web on turn 2 by outspeeding the threat with chlorophyll. Flail, X scissor, Reflect or Lightscreen could fill the remaining move slots for additional support options.
Magcargo is a nice option for Sun teams.
Its typing helps guard the grass types on the team from incoming Fire Attacks with a 4x resistance as well as tackling Flying type attacks. Shell Smash also enables it to shell out (pun totally intended) hard hits against a multitude of types with Earth Power, Ancient Power, and Lava Plume, which has a 30% burn chance.
Corsola can surprisingly fit on Sun teams as well. This Water/Rock type absorbs Fire, Rock, and Flying attacks with ease. It is capable of reliably setting up sun and recovering health with both Recover and its ability Regenerator. It also can set up Stealth Rock.
Another (gimmicky) option could be to exploit its ability Hustle, pairing it with Sucker Punch or even Explosion but be prepared to miss...
Flareon can pull off some amazing feats when given enough speed from a Flame Charge. Flare Blitz gets a boost from the Sun and from either of Flareons abilities. Flash Fire helps protect your Grass Types, but Guts provides better offensive power to moves such as Facade which helps out Castform take out special walls. Even some physically defensive Pokemon will have a hard time with Flareon's barrages. Just remember that a Guts Flareon is on a timer which drops Faster with repeated use of Flare Blitz.
NU
Shiftry does great work in Sunlight, despite its dark tendencies. It has the option of being either physically or specially offensive with access to Swords Dance and Nasty Plot. Growth combines the best of both worlds, giving you a mixed sweeper. Knock off is great for obvious reasons, and access to Leaf Blade makes this a better version of Leafeon. Sucker punch, Tailwind, Dark Pulse, and Leafstorm are just some of the other great moves this Pokemon has tucked away.
Cradily loves sunlight. Synthesis restores its hp by 3/4 of its max with the sun up. Storm Drain keeps water attacks from dousing your Fire type Pokemon, Toxic and Stockpile help stall out walls and Giga Drain gives it an additional recovery move as well as a form of attack.
HAIL
If you love Castform, then for the love of... Castform,
Do Not Use Hail
You will doom its performance as it hardly benefits from this weather. It eliminates the novelty of Weather Ball since Blizzard will have higher base power, perfect accuracy, and a 10% chance to freeze the opponent. You will also lose access to reliable, stronger coverage options, forcing you to use moves such as flamethrower and thunderbolt, which is a significant problem since Castform has less than decent special attack. Not to mention the amount of weaknesses that comes with your new typing severely cripples your chances of success.
Possible Teammates
If you are reading this and plan to create a hail team in spite of my warning, you probably do not need help creating a team... That or you have no sense. In any case, I will provide a few Pokemon worth consideration.
PU
Avalugg will wall ALOT of physical hits.
Although it is best suited with Sturdy and Mirror Coat, Ice Body and Recover allow it to negate almost any physical attack. While you can tank some super effective attacks, do not carelessly throw Avalugg into super effective hits and keep an eye out for Toxic. Do not stay into ANY special attacks, not even resisted ones.
Regice picks up where Avalugg leaves off. Regice can take a beating from most special attacks and makes use of its hidden ability Ice body. Confuse Ray, Iron Head and Thunder Wave gives it that extremely annoying Parafusion combo. It also can make for a startling special attacker using Lock On with Zap Cannon and Blizzard when Hail is not up.
Walrein takes care of most special attacks it isn't weak to and finds use in all its abilities. Ice Body provides some recovery, Thick Fat boosts its resistances which helps it take on Fire types, and Oblivious prevents it from being Taunt bait. It can be a great Hail inducer, and puts a heavy dent into almost anything that isn't a Ghost type with Super Fang and Toxic (Dont forget hail damage). Stockpile boosts its ability to wall.
NU
Abomasnow... Good for setting up hail with its ability Snow Warning...
Thats about it... Its weak to SEVEN of the Pokemon types... Safe to say most Pokemon have a coverage move that will destroy it.
It is much harder to use than Castform but instant Hail means you have a free turn to
use Blizzard or Wood Hammer. It is the only Pokemon of the four Ice types in NU worth noting...
Final Tips
Keep in mind Castform's stats are not too great so special defensive Pokemon, regardless of weaknesses, are going to do a fair job at walling it and any physical Pokemon with decent speed investment will overpower it. Also, be wary of set up Pokemon, who will score a few stat boosts if you are careless. Items that help Castform the most are; Choice Specs, Assault Vest, Choice Scarf, Life Orb.
Castform generally works best in teams closer to its tier, however it can succeed in higher tiers with the appropriate team. Castform is more successful when you use it earlier in the match. Using it late game could be fatal if you lose your weather inducers.
Hopefully this was useful to someone!
