Roserade's 4 weaknesses are Fire, Ice, Flying and Psychic, so a rock move would be good! I'm assuming you're going for special sweeper here.
She doesn't learn sandstorm so you'll need Tyranitar or Hippowdon to set it up with Sand Stream (another pokemon with sandstorm works but with Sand Stream it's more permanent). Weather Ball would probably be my choice, for 100 power instead of 30-70 like HP Rock. Holding Leftovers is useful to negate sandstorm damage.
She doesn't really have the defenses for Toxic Spikes (especially losing damage from sandstorm) but it's certainly a viable option to throw in if you're able to predict the opponent switching or not hitting you hard.
Personally I would go for Sludge Bomb as a powerful STAB move with mostly neutral coverage and in case no weather condition or the wrong one is active.