Let's face it, if you're using Castform at all, it's for the pure novelty value of Weather Ball. Having Castform on your team and not using Weather Ball is like visiting Paris without going to the Louvre, visiting New York without eating a hot dog, or visting Johannesburg without setting a car on fire. Start by using the weather move of your choice, and then once the skies have changed, it's time to pound away at the opponent with Weather Ball. Thunderbolt and Ice Beam are there to provide additional coverage, but you'll be so busy clicking Weather Ball you won't even realize these additional moves are there and that Castform got KOed 3 turns ago.
Whether Castform the Weather Pokemon can weather the weather is dependent on whether the weather that it's weathering is the proper weather for it to weather. Whether the opponent can weather the weather is also subject to whether the weather that it's weathering is the right weather to weather. Whether Castform can weather the opponent is once again dictated by the weather and whether that weather is the favorable weather for weathering. But most importantly, you should know whether there will be weather to weather rather than what weather they will be weathering and whether they can weather that weather.
Castform's overall low stats means that it'll struggle against many foes, so it'll appreciate support from entry hazards, dual screens, paralysis, and you returning it to its Poke Ball where it can take shelter for the rest of the battle. The items and EVs chosen ensure that Castform hits as hard as possible with Weather Ball, but odds are at the end of the day it's still going to be the equivalent of a wet paper towel fighting a mountain lion, even if it looks like Castform's got the balls to pull it off. Hopefully you'll know better next time.
