Breloom's good, but it's really REALLY predictable. You should expect a Sub-Spore-Puncher. Its last move will either be Mach Punch or Seed Bomb, and its item will be Toxic Orb so it can recover more and more HP every turn. If you know the standard moveset and its strategy, just switch accordingly. I've made my friend's Breloom run out of PP by switching back and forth between Dusknoir, Vensusaur, and Salamence. Dusknoir wasn't affected by Focus Punch, both Seed Bomb and Focus Punch do hardly anything to Venusaur, and if Substitute is down on Breloom, then Salamence's Intimidate lowered Breloom's Attack. I knew I didn't have a good Pokemon to take his Breloom one-on-one, so I just predicted him and switched a ton.
So basically, Breloom's more predictable than...Melee Bowser.