you have to use the shiny stone. BEWARE, only use the shiny stone on a high level, cause once you've used the stone you can't increase stats!
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All you need to do to evolve a Roselia is use a shiny stone on it. No trading, and it will never evolve on it's own no matter what level you get it to.
Roselia only evolves in Pokémon Platinum, Diamond, Pearl, HeartGold and SoulSilver. In order to evolve Roselia, you have to use a Shiny Stone on it, it doesn't evolve by any level.
There is no specific level that a Budew will evolve at. However, if you level it up on high happiness, you will end up with a Roselia. And if you use a Shiny Stone on the Roselia, you will get a Roserade.
With a Shiny Stone
Shiny Stone