So, it seems your Butterfree has Modest as its' Nature. As for a set, I have been tinkering all kinds of different ideas for sure to make use of Butterfree. Here's one I just had in mind
Nature: Modest (Special Attack up, Attack down)
EVs: Emphasis on Special Attack, HP, and Speed.
Attacks: Bug Buzz/Psychic/Energy Ball/Quiver Dance
Held Item: Wise Glasses
This would be a set mostly to have Butterfree get in with some quick special attacks and sweep them. With this set, Butterfree'd cover Grass, Psychic, Dark, Fighting, Poison, Water, Rock, and Ground, with Butterfree having a quadruple weakness to Rock, so that helps.
Here is another Set
Nature: Timid (Speed up, Attack down)
EVs: Emphasis on Speed, Special Attack, and HP
Attacks: Bug Buzz/Sleep Powder/Dream Eater/Quiver Dance
Held Item: Big Root
Put the enemy to sleep with Sleep Powder thanks to its accuracy going up due to Compoundeyes, as well as the Big Root giving you more HP from Dream Eater than normal. Bug Buzz for Same Type Attack Bonus, as well as Quiver Dance to raise Special Attack, Special Defense, and Speed so Butterfree can do more damage with Bug Buzz and Dream Eater, as well as perhaps the pre-emptive strike too.