Roserade is basically another typical Grass type. The only thing that really makes it stick out is the fact that it has Weather Ball and Technician.
Besides that, there's really no reason to use it over other Grass types. Its movepool is basically Hidden Power, Shadow Ball, Weather Ball, Sludge Bomb, Extrasensory (?), and one of its many Grass type attacks.
Stat wise, its ok. It has below average attack, nice Special Attack, and decent Speed.
Natural Cure: Cures status conditions whenever you switch out. Not bad.
Poison Point: 30% chance of being poisoned upon physical contact. Not bad either.
Technician(DW) This is where Roserade really shines. This ability (which I can't seem to spell correctly on my first attempt) boosts all attacks with a base power of 60 or lower by 1.5 times. So an attack such as Weather Ball, which normally has a base power of 50, would have a base power of 75 with this ability. Really freaking good.
Roses are prettiest when in the Sun
-Solarbeam
-Weather Ball
-Hidden Power [Ground]
-Shadow Ball
Item: Choice Scarf
Ability: Technician/Natural Cure
Nature: Modest (+Sp. Atk, -Atk)
4 Hp/252 Sp. Atk/252 Speed
When Technician is released, use that. Until then, I suggest Natural Cure, as it will be switching out often anyway. Because this should be used in the Sun, Solarbeam is necessary. Weather Ball becomes a Fire type attack, so nice coverage. Hidden Power Ground is for Fire types. Shadow Ball for coverage. I chose Modest over Timid because with Max Speed EVs and IVs without Timid and with Scarf, its Speed peaks at 418.
Cleric
-Aromatherapy
-Giga Drain
-Toxic Spikes
-Sleep Powder/Spikes
Item: Leftovers
Ability: Natural Cure
Nature: Timid (+Speed, -Atk)
252 Hp/ 4 Sp. Def/252 Speed
This set is basically to heal your team of status conditions and possibly setting up hazards. Giga Drain is to survive longer, while Sleep Powder to stop sweeps or waste a berry.
Roses are red, like BLOOD
-Energy Ball/Magical Leaf
-Hidden Power [Fire]
-Shadow Ball
-Growth/Weather Ball
Item: Life Orb
Ability: Natural Cure/Technician
Nature: Timid
4 Hp/252 Sp. Atk/252 Spd
This is a basic sweeper set. Energy Ball if you have Natural Cure, Magical Leaf with Technician. Shadow Ball and HP Fire for coverage. If you have Technician, choose Weather Ball. If not, I would go with Growth for the extra power.
Other Options
Sludge Bomb serves as high-powered STAB.
Giga Drain over Energy Ball if you want to live longer.
Extrasensory can do some damage, but it has absolutely no good coverage.
Mega Drain can be stronger than Giga Drain with Technician.
Double and Triple Options
Don't.
Partners
Ninetails is nice for Weather Ball, Politoed to lower super-effective Fire attacks. You want to take out Special Walls, so pokemon who can take out the common ones like Conklederp or Dragonite are good. Also, if you want to set up hazards, spin-blockers are good.
Countering Roserade
Physical Fire types kill it, pokemon with high Special Defense and decent Hp wall it. Simple as that.