The best moves for Castform (Sunny) are Ember and Weather Ball when attacking Pokémon in Gyms. This move combination has the highest total DPS and is also the best moveset for PVP battles.

Castform (Sunny) currently has no evolutions in Pokémon GO.

There are 4 different forms of Castform:

There is a very low chance that you can get a shiny Castform, which has the following appearance:

Summary

Castform (Sunny) is a Fire Pokémon. It is vulnerable to Ground, Rock and Water moves. Castform's strongest moveset is Ember & Weather Ball and it has a Max CP of 1,632.

About

"Castform's appearance changes with the weather. This Pokémon gained the ability to use the vast power of nature to protect its tiny body."