Rillaboom @ No Item / Grassy Seed
Ability: Grassy Surge
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Acrobatics
- Swords Dance
- Grassy Glide
- Superpower / Wood Hammer
Fairly simple really, unboosted Acrobatics has a decent chance to OHKO outright (252+ Atk Rillaboom Acrobatics (110 BP) vs. 224 HP / 0 Def Buzzwole: 384-456 (93.4 - 110.9%) -- 62.5% chance to OHKO) and most definitely will after a little chip (U-turn, Hazards etc). The other moves depend on the rest of your team; if you have a Magnezone, SD + dual grass STAB is a great way to go as this makes it easier to break through other walls such as Pex or Magic Guard Clef (though Superpower is still good if you also want to lure/beat Heatran). SD also allows you to straight up beat Tangrowth with a +2 Acrobatics after chip, though Rillaboom can usually take whatever Tang dishes out and beat it 1 on 1 with this set regardless (Standard Tangrowth only really has Focus Blast to chunk Rillaboom, and Sludge Bomb isn't commonly used)
While Grassy Seed is technically better in every way to no item, running such could alert the opponent that you potentially have Acrobatics, while with no item you're able to bluff a whole slew of other sets before you attack. In some scenarios, giving away the set couldn't matter much however (they get swept regardless of the bluff, the opponent scouts the set) so running Grassy Seed is still a good option to potentially gain additional setup situations due to the defence buff (e.g being able to take Landorus U-turn, Weavile Ice Shard, Ferrothorn Body Press/Gyro Ball better than otherwise)