I like a lot of the sets on here, but I was thinking of something like this:
EVs: 252 Attack, 252 Speed, 4 Health
Item: Choice Scarf
Wearing a Choice item with full speed investment many opponents won't expect being outsped. With Power-up Punch and the speed, this Breloom can snowball if left unanswered. Playing around Talonflame, Crobat, and ghost types is important, hence the Stone Edge.
Mach Punch may seem weird, but with full speed investment the biggest concern could be priority moves (Talonflame, Weavile, Mamoswine, etc.) and in priority battles speed wins again. Choice Scarf Breelom with max speed hits 393 speed, outspeeding everything without a scarf or speed boosts. Against super effective priority users, having Mach Punch could net valuable kills on these targets.