Well, Roserade has respectable Special Attack and okay Speed, but its low Defense and HP stat leave it vulnerable to physical attackers. It's mostly your typical Grass type, although its new Dream World ability is interesting.
Ablities:
Natural Cure: Not bad, but Roserade can't be Poisoned anyway, and Burn only affects the physical Attack stat, but it's nice for recovering from Thunder Wave and the occasional Hypnosis.
Poison Point: 30% chance for Poisoning the opponent on direct contact is pretty useless most of the time, and even then, Roserade's 55 base Defense doesn't help it.
Technician: Boosts 60 or less base power moves by 1.5x. Remember how Giga Drain went from 60 to 75 base power? An improvement for most Grass types, but looks like Technician Roserade just missed its chance. However, it has its use with Magical Leaf.
Generic Grass Type (AKA SubSeed)
- Giga Drain
- Leech Seed
- Substitute
- Synthesis/Sleep Powder/Toxic
Item: Leftovers
Ability: Natural Cure
EVs and Nature: Bold (+Def, -Atk) or Modest (+SpAtk, -Atk); 252 Def/56 SpAtk/200 SpDef
In the rare event that someone reading this doesn't know about SubSeed sets, it's basically lasting as long you can with Substitute and Leech Seed and whatever other moves you have while Leech Seed slowly whittles down their HP. Synthesis is good, but not necessary when you have Leftovers, Giga Drain, and Leech Seed. Sleep Powder forces your opponent to sit there and watch their HP get sucked out. Toxic is for killing your opponent faster.
Spike Sweeper
- Spikes
- Toxic Spikes
- Giga Drain
- Venoshock
Item: Leftovers
Ability: Natural Cure
EVs and Nature: Bold (+Def, -Atk) or Modest (+SpAtk, -Atk); 252 Def/200 SpAtk/56 SpDef
Set up the entry hazards and let Venoshock destroy anything that switches in. Giga Drain and Leftovers help you last longer.
Sweeping in the Sun
- Sunny Day
- Solarbeam
- Synthesis
- Shadow Ball
Item: Heat Rock
Ability: Natural Cure
EVs and Nature: Modest (+SpAtk, -Atk); 4 HP/252 SpAtk/252 Spd
Use Sunny Day to amp up Solarbeam and Synthesis. Shadow Ball will take care of Psychic types. Heat Rock will let Sunny Day last an extra 3 turns. This set would've been a lot better if Roserade had Chlorophyll.
Technician
- Magical Leaf
- Swift/Round
- Weather Ball
- Rain Dance
Item: Wise Glasses/Damp Rock
Ability: Technician
EVs and Nature: Modest (+SpAtk, -Atk); 4 HP/252 SpAtk/252 Spd
I know this set is not that great looking at first, but Roserade doesn't have that many moves that go with Technician. With Technician, Magical Leaf and Swift turn into their respective type equivalents of Aura Sphere. Swift can be replaced by Round if you have some sort of Doubles/Triples strategy with Round. Weather Ball + Rain Dance + Technician = a Water type move with a base power of 225 (including Rain boost to Water type moves), so that will take care of any Fire types. Personally, I would give this set Wise Glasses, but I can understand the allure of a 225 base power move, so go ahead with a Damp Rock if you want to.
Weather Ball Sweeper
- Sunny Day
- Rain Dance
- Weather Ball
- Synthesis/Growth
Item: Leftovers/Metronome/Heat Rock/Damp Rock
Ability: Technician
EVs and Nature: Modest (+SpAtk, -Atk); 4 HP/252 SpAtk/252 Spd
Pick a weather and start sweeping with your 225 base power Weather Ball. Synthesis will let you heal in Sunny weather. Did you know, however, that in 5th gen, Growth increases both offensive stats two stages in Sunny weather? It makes Growth a viable option. Leftovers helps you last longer, Metronome (yes, they made an item with that name, look it up) will power up Weather Ball each consecutive time you use it, or a weather rock will help one type of weather last longer (Sun for Synthesis/Growth or Rain for murdering Fire types).
Support Set
- Aromatherapy
- Sleep Powder
- Toxic
- Venoshock
Item: Leftovers
Ability: Natural Cure
EVs and Nature: Bold (+Def, -Atk) or Modest (+SpAtk, -Atk); 252 Def/56 SpAtk/200 SpDef
I know it's not wonderful or anything, since Roserade is more suited to be a sweeper. Aromatherapy cures all status problems on your team, so it's pretty amazing. It's mostly the reason why this set is possible. Sleep Powder is probably better on Prankster Whimsicott, but it works well enough with Roserade in Doubles/Triples. The combination of Toxic + Venoshock gives a good status problem with a move that has poor typing (Poison isn't the best type to make supereffective hits) but an impressive 130 base power (after Toxic hits).
Other Options
There are a few worth mentioning, but nothing too special.
Hidden Power can actually work well with Technician, but if you're like me, you may be too lazy to go try and get the right IVs.
Hyper Beam is something that can work. You never know when you'll need a Normal type move with 150 base power.
Double Team; Roserade has the speed to try it, though you have to watch out for Physical moves if you're going to try to use it multiple times.
Sludge Bomb is a powerful move with STAB, but being a Poison type makes it questionable to use.
Grass Knot is a possibility on most Grass types. Go ahead if you want to use it.
Water Sport halves Fire type move damage. That's good, but Rain Dance already covers that.
Stun Spore can be used instead of a Sleep Powder. I prefer inflicting Sleep over Paralysis with Roserade, but that's just me.
Petal Dance has the power of Solarbeam without the need for Sun. Just watch out for when you get the Confusion (or your opponent could switch in the middle of it).
Extrasensory is a move with 80 base power other than Shadow Ball.
Leaf Storm is stronger than Solarbeam and Petal Dance, but be prepared to switch out after that 2 stage Special Attack decrease.
Worry Seed changes the target's ability to Insomnia. Use it to prevent Rest.
Double and Triple Battle Options
Roserade has Aromatherapy. That's already good enough a reason. However, it also has Worry Seed. Try using that on your partnered Slaking or Archeops.
Partners
Like aforementioned, anything that benefits from Worry Seed. Drought Ninetales is a good partner as well (for what Grass type is DroughtTales not, anyway?).
Countering Roserade
Take advantage of its poor HP and Defense and use a powerful physical move. The tricky thing about Roserade is that most of the type weaknesses it has mostly are made up of Special moves and it has a surprising 105 Special Defense base stat. Ice, Psychic, and Fire; they're mostly made of Special moves. Flying type moves, however, mostly make up Physical moves, and in addition, Flying type Pokemon aren't affected by the Spikes Roserade can set up. Speaking of which, you can use a Rapid Spinner to get rid of those Spikes. A weather changer can mess up a Weather Balling Roserade.
Ablities:
Natural Cure: Not bad, but Roserade can't be Poisoned anyway, and Burn only affects the physical Attack stat, but it's nice for recovering from Thunder Wave and the occasional Hypnosis.
Poison Point: 30% chance for Poisoning the opponent on direct contact is pretty useless most of the time, and even then, Roserade's 55 base Defense doesn't help it.
Technician: Boosts 60 or less base power moves by 1.5x. Remember how Giga Drain went from 60 to 75 base power? An improvement for most Grass types, but looks like Technician Roserade just missed its chance. However, it has its use with Magical Leaf.
Generic Grass Type (AKA SubSeed)
- Giga Drain
- Leech Seed
- Substitute
- Synthesis/Sleep Powder/Toxic
Item: Leftovers
Ability: Natural Cure
EVs and Nature: Bold (+Def, -Atk) or Modest (+SpAtk, -Atk); 252 Def/56 SpAtk/200 SpDef
In the rare event that someone reading this doesn't know about SubSeed sets, it's basically lasting as long you can with Substitute and Leech Seed and whatever other moves you have while Leech Seed slowly whittles down their HP. Synthesis is good, but not necessary when you have Leftovers, Giga Drain, and Leech Seed. Sleep Powder forces your opponent to sit there and watch their HP get sucked out. Toxic is for killing your opponent faster.
Spike Sweeper
- Spikes
- Toxic Spikes
- Giga Drain
- Venoshock
Item: Leftovers
Ability: Natural Cure
EVs and Nature: Bold (+Def, -Atk) or Modest (+SpAtk, -Atk); 252 Def/200 SpAtk/56 SpDef
Set up the entry hazards and let Venoshock destroy anything that switches in. Giga Drain and Leftovers help you last longer.
Sweeping in the Sun
- Sunny Day
- Solarbeam
- Synthesis
- Shadow Ball
Item: Heat Rock
Ability: Natural Cure
EVs and Nature: Modest (+SpAtk, -Atk); 4 HP/252 SpAtk/252 Spd
Use Sunny Day to amp up Solarbeam and Synthesis. Shadow Ball will take care of Psychic types. Heat Rock will let Sunny Day last an extra 3 turns. This set would've been a lot better if Roserade had Chlorophyll.
Technician
- Magical Leaf
- Swift/Round
- Weather Ball
- Rain Dance
Item: Wise Glasses/Damp Rock
Ability: Technician
EVs and Nature: Modest (+SpAtk, -Atk); 4 HP/252 SpAtk/252 Spd
I know this set is not that great looking at first, but Roserade doesn't have that many moves that go with Technician. With Technician, Magical Leaf and Swift turn into their respective type equivalents of Aura Sphere. Swift can be replaced by Round if you have some sort of Doubles/Triples strategy with Round. Weather Ball + Rain Dance + Technician = a Water type move with a base power of 225 (including Rain boost to Water type moves), so that will take care of any Fire types. Personally, I would give this set Wise Glasses, but I can understand the allure of a 225 base power move, so go ahead with a Damp Rock if you want to.
Weather Ball Sweeper
- Sunny Day
- Rain Dance
- Weather Ball
- Synthesis/Growth
Item: Leftovers/Metronome/Heat Rock/Damp Rock
Ability: Technician
EVs and Nature: Modest (+SpAtk, -Atk); 4 HP/252 SpAtk/252 Spd
Pick a weather and start sweeping with your 225 base power Weather Ball. Synthesis will let you heal in Sunny weather. Did you know, however, that in 5th gen, Growth increases both offensive stats two stages in Sunny weather? It makes Growth a viable option. Leftovers helps you last longer, Metronome (yes, they made an item with that name, look it up) will power up Weather Ball each consecutive time you use it, or a weather rock will help one type of weather last longer (Sun for Synthesis/Growth or Rain for murdering Fire types).
Support Set
- Aromatherapy
- Sleep Powder
- Toxic
- Venoshock
Item: Leftovers
Ability: Natural Cure
EVs and Nature: Bold (+Def, -Atk) or Modest (+SpAtk, -Atk); 252 Def/56 SpAtk/200 SpDef
I know it's not wonderful or anything, since Roserade is more suited to be a sweeper. Aromatherapy cures all status problems on your team, so it's pretty amazing. It's mostly the reason why this set is possible. Sleep Powder is probably better on Prankster Whimsicott, but it works well enough with Roserade in Doubles/Triples. The combination of Toxic + Venoshock gives a good status problem with a move that has poor typing (Poison isn't the best type to make supereffective hits) but an impressive 130 base power (after Toxic hits).
Other Options
There are a few worth mentioning, but nothing too special.
Hidden Power can actually work well with Technician, but if you're like me, you may be too lazy to go try and get the right IVs.
Hyper Beam is something that can work. You never know when you'll need a Normal type move with 150 base power.
Double Team; Roserade has the speed to try it, though you have to watch out for Physical moves if you're going to try to use it multiple times.
Sludge Bomb is a powerful move with STAB, but being a Poison type makes it questionable to use.
Grass Knot is a possibility on most Grass types. Go ahead if you want to use it.
Water Sport halves Fire type move damage. That's good, but Rain Dance already covers that.
Stun Spore can be used instead of a Sleep Powder. I prefer inflicting Sleep over Paralysis with Roserade, but that's just me.
Petal Dance has the power of Solarbeam without the need for Sun. Just watch out for when you get the Confusion (or your opponent could switch in the middle of it).
Extrasensory is a move with 80 base power other than Shadow Ball.
Leaf Storm is stronger than Solarbeam and Petal Dance, but be prepared to switch out after that 2 stage Special Attack decrease.
Worry Seed changes the target's ability to Insomnia. Use it to prevent Rest.
Double and Triple Battle Options
Roserade has Aromatherapy. That's already good enough a reason. However, it also has Worry Seed. Try using that on your partnered Slaking or Archeops.
Partners
Like aforementioned, anything that benefits from Worry Seed. Drought Ninetales is a good partner as well (for what Grass type is DroughtTales not, anyway?).
Countering Roserade
Take advantage of its poor HP and Defense and use a powerful physical move. The tricky thing about Roserade is that most of the type weaknesses it has mostly are made up of Special moves and it has a surprising 105 Special Defense base stat. Ice, Psychic, and Fire; they're mostly made of Special moves. Flying type moves, however, mostly make up Physical moves, and in addition, Flying type Pokemon aren't affected by the Spikes Roserade can set up. Speaking of which, you can use a Rapid Spinner to get rid of those Spikes. A weather changer can mess up a Weather Balling Roserade.