I used to use a Breloom while playing on PO. When I used Breloom, my moveset was Drain Punch, Mach Punch, Bullet Seed, and Spore (although I did hear that Spore was uber). Since it was PO, I had Breloom get the ability Technician, which powers up moves with a base power of 60 or less. It held a Metronome. With maxed out speed and attack and a Jolly nature, it outsped most pokemon. You could also take some off attack and invest in HP for more hits.
If they hit first, Drain Punch would regain the lost health. Remaining HP could be picked off with Mach Punch.
Bullet Seed is Grass Type physical attack. That means it gets STAB, multiple hits, Technician power up, and if used twice in a row (on two turns in a row) it gets a boost from the Metronome. Same thing goes for Drain Punch and Mach Punch.
You can also switch in and use Mach Punch for revenge killing.
Breloom is a great pokemon, it's one of my favorite. If you don't have Technician, then i would suggest Poison heal and a Toxic orb instead, and Seed Bomb instead of Bullet Seed.