first of all, it will only work on soul silver or heart gold.
on one of those games take a male roserade that knows weather ball via the move tutor and a female victreebel, bellsprout or weepinbell of any kind and put them both in the day care. once an egg is hatched, it will know weather ball, as well as a few other attacks
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to teach Roselia weather ball, you must go to the move tutor in pastoria- but make sure you have a heart scale, which can be found underground.
you have to get a heart scale and go to the move tourtor andmake it learn it
Roserade doesn't exactly learn Weather Ball in the tradition sense but it does know and can learn the move. In-order to teach Roserade the move Weather Ball you need to find a Move Reminder which is an NPC that will reteach a Pokémon moves it already learned at lower levels for the cost of 1 Heart Scale. This is the only way to teach Roserade Weather Ball, many other Pokémon also have moves that require a Move Reminder to learn.
Victreebel does not have an evolution. It is the second and final evolution of Bellsprout. Nothing comes after Victreebel.
You have to catch a Bellsprout or a Weepinbell (Evolves from Bellsprout). Then you can use a Leaf Stone on the Weepinbell and it will become a Victreebel.