Castform learns the move Weatherball, at level 30. This is the last move it can learn.
1St move is Weatherball Depending on what the weathers like
You have to learn Weather Ball from the Move Tutor. You will need a heart scale though which you can get from underground.
level 50
Castform does not learn Thunder as it levels up. Thunder is a TM that you can teach it. (In Pokemon Emerald, Ruby, or Saphire, you can get Thunder in the Lilycove Department Store.)
level 30
No, I don't think so.
Its body changes with the weather, and that is the way it reacts and moves.
No, castform morphs due to the weather.
By using Sunny day, Hail, Rain Dance. Moves that It can learn
Castform does not evolve. However it will change it's appearance according to the weather.Sunny-Castform turns red and looks like a flower.Raining-Castform turns blue and looks like a raindrop.Hailing-Castform turns a blue and has grey cloud like shapes around it's body.Normal-If no abnormal weather conditions are present during the battle, Castform remains in it's original form.
You can't get Castform in SoulSliver without trading or migrating.
Castform does not transform with the Mystic Water. Castform's forms are entirely based on the weather effect currently active in battle.