Butterfree, evolved from Caterpie, is one of the most overlooked bug types in Pokemon. Given the right moves, it can be devastating to all types of enemies. On its own, Butterfree is incredibly useless, but when put into a team, it can destroy.
The Team and Move set
So the first part of this strategy is getting a strong team set up around Butterfree. You want as many strong attackers, physical and special, as possible. Make sure you also get decent type coverage. Meganium, Pidgeot, Golem, and Ampharos are all good choices. Just make sure you don't have too many support Pokemon in there.
The second part is all about Butterfree. There are several movesets that would work, but the one that I've found to work best consists of the following:
Safeguard - Poison Powder - Tailwind - Silver Wind
Safeguard is a great move to have when fighting against pokemon that can inflict harmful status conditions, notably paralysis and burns. Tailwind helps to boost the team, because after all, Butterfree is a support Pokemon. Poison Powder is in there to put out some damage when masses of smaller enemies come rushing in. And finally, Silver Wind is a great move for Butterfree, because as well as being a decent STAB attack, it has a chance to boost all 5 of the user's stats.
Thank you for reading, and I hope this helped!