I never liked Bellossom so much. Is just outclassed. But still usable, so let's go. ;182;
Set name: Double status
Hold item: Leftovers
Nature: Calm
Ability: Chlorophill
EVs: 252 Hp, 252 SpDef, 4 Def
-Sleep Powder | Stun Spore
-Toxic/Stun Spore | Teeter Dance
-Moonlight/Hidden Power [Ice/Fire/Ground/Fighting] | Substitute
-Giga Drain
Standard bulky grass stuff. Very common, very straightforward. Double status is a pretty effective strategy, so let's pull it off. Sleep Powder gives you some kind of niche, as it sets you apart from Meganium and Shaymin, who have to rely on the unreliable Grasswhistle. Usable pure Grass types learning Sleep Powder are Bellossom, Doredia, Tangela and Tangrowth. Doredia doesn't have great defensive stats, while the others are more phisically defensive, so a specially defensive Bellossom isn't completely outclassed. As you are using double status, you will want either Toxic or Stun Spore. They are both good, choose depending on what your team needs. Moonlight recovers your HP and is amazing when the sun is up.
If you feel Moonlight unreliable, go with your Hidden Power of choice. HP [Ice] is for grass, flying and dragon types, HP [Fire] covers grass, bug and steel types but is crippled by the rain, and rain is one of the reasons not to use Moonlight. If you're using Toxic, is recomended to pick an HP super effective against steels (and poisons to a lesser extent) as they are immune to it. So pick HP [Fire/Ground/Fighting] depending on what pokemon you feel more threatening. Giga Drain is your best attacking move. Good power, awsome effect, good accuracy... just good. Instead of Sleep Powder and Toxic/Stun Spore you can pick Stun Spore and Teeter Dance. This gives you the para-fusion combo, wich is very good. In this case, you can use substitute on the third slot as you will force tons of switches and the opponent will hardly be capable of attacking you, and Giga Drain can recover the HP loss. For the EVs specially defensive is recomended, so Calm Nature whith 252 HP, 252 SpDef, 4 Def. Healer is a useless ability in single battles, so, unless you are doing doubles/triples, go with Chlorophill.
Other Options
Pretty much nothing... you could use phisically based attacking moves. Leaf Blade and Drain Punch/Nature Power (wich becomes Earthquake on Wi-Fi). Your base Attack stat is only 10 points lower than your Special Attack, but those moves are more powerful, so they are actually usable, but probably STAB Giga Drain is what you will want mostly.
Partners
Standard stuff that usually partners grass types. You can pick a fire and a water pokemon to make the good fire/grass/water defensive core.
Counters
Espeon and Xatu pack Magic Bounce to bounce back your status moves rendering you less threatening. A double status set isn't easily counterable, as it can cripple it's counters, just don't let him put to sleep your main counter.
Set Name: What every single grass type does
Hold item: Leftovers
Nature: Calm
Ability: Chlorophill
EVs: 252 Hp, 92 SpDef, 164 Spe
-Substitute
-Leech Seed
-Hidden Power [Fire/Ice]
-Giga Drain/Sleep Powder
Very original. Outclassed. Use this in the sun. Leech Seed is a 3rd gen event move, so I don't know about if it's legal with Sleep Powder, but if so, use Sleep Powder. Hidden Power is for grass types that are immune to Leech Seed. 164 Speed EVs to outspeed base 111s under the sun. You can invest less in speed and more in bulkyness, so think about what you want to outspeed. and by the way, just use Tangrowth/Eviolite Tangela.
Set Name: Phisical sweeper Bellossom?
Hold item: Life Orb
Nature: Adamant
Ability: Chlorophill
EVs: 252 Atk, 252 Spe, 4 HP
-Swords Dance
-Leaf Blade
-Nature Power/Drain Punch
-Sleep Powder/Return/Double Edge
The only sweeping set for Bellossom that carries a boosting move, and it's a good one. Under the sun you reach 398 Speed, so you outspeed almost anything unboosted. Sleep Powder gives you a free turn to set up a Swords Dance. Leaf Blade is a very good STAB, while Nature Power (EQ) or Drain Punch add useful coverage. You can forego Sleep Powder and use a move to improve your coverage. Double Edge works well alongside Drain Punch hitting Flying, Bugs and Poison types hard, while Drain Punch softens the recoil. Double Edge is illegal with Nature Power, so if you opt for it you can only use Return.
Obvious sweeping EV spread, Adamant and Life Orb as you need the power.
Other Options
Tangrowth is better. You only have Drain Punch to differentiate a bit while it has Brick Break.
Partners
Something to weaken your counters. Rock complements well your two main attacking moves, so Stealth Rock are appreciated for Fire, Flying and Bug types. Rock types have good sinergy both defensively and offensively. Kabutops brillantly resists you weaknesses and easilly takes out you counters. A fire type is appreciated to kill steels.
Counters
Without a normal move, Fliyng and Bug types eat you for breakfast. Standard Walls (Skarmory, Toxic Orb/Fling/Acrobatics Gliscor, Forretress, Ferrothorn) own you no matter what. Bulky grass types are problematic too, expecially those carrying HP [Fire].
