It would depend on what set Breloom is running, but for the most part competitive players always use Spore. Aside from that, it's either Toxic Heal stall or a Technician set. If your main problem is Spore, use something like Venusaur (mega or normal) or even Amoongus/ Vileplume (if you want more variety) to tank most of Breloom's hits and deal x2 with Sludge Bomb.
For the Toxic healers, it's a good idea to have something with Infiltrator, because Toxic Heal sets like to run Subsitute (which the ability will bypass). 1 Air Slash from Noivern will OHKO Breloom. Other options include Infiltrator Chandelure, or Hyper Voice on Mega Gardevoir/ Mega Altaria; both these megas have Pixilate and will outspeed Breloom at 252 EVs and 31 Speed IVs.
Also Talonflame is more than viable but either spikes or rocks have to be up on the field, because Technician Breloom usually hold Focus Sash.
Burn that double kangaroo before it has a chance to do anything. Shuts down most of Kangkaskhan's moves (Power-Up Punch, Fake Out, Return, & Sucker Punch as long as you don't attack). Kangaskhan will only be able to hit you with something like Crunch or Earthquake after that. Only thing is to be careful with Scrappy and don't send Sableye too soon before it mega evolves.
Sableye @ Leftovers
EVs: 252 HP, 136 Sp. Def, 120 Def
- Knock Off/ Foul Play