Halcyon.
@Choice Specs
Physically Defensive Celebi stops any Breloom set in it's tracks.
Celebi @ Leftovers
Nature: Bold
Ability: Natural Cure
EVs: 252 HP / 232 Def / 20 Spe
• Giga Drain
• Psychic
• Recover
• Thunder Wave/Baton Pass
Any Celebi will wall Breloom to hell and back, but this one especially will demolish him. It doesn't have the best coverage with the given moves, but honestly you can change them to whatever you like, Breloom still isn't getting through anytime soon. Celebi is "immune" to sleep, since switching out will heal you, and is also immune to the rare sub/seed Breloom variant's Leech Seed. A specially defensive set can also be used in order to act as a Keldeo counter as well, because even with special bulk, it would take a Swords Dance and 5 Bullet Seed hits to do serious damage to Celebi, and either way, she can OHKO back with Psychic. Being a counter to two of the tier's top threats is nothing to scoff at, and Celebi is easy to fit onto many teams.
Here are some calcs
252+ Atk Breloom Focus Punch vs. 252 HP / 228+ Def Celebi: 99-117 (24.5 - 28.96%) -- possible 5HKO
252+ Atk Life Orb Technician Breloom Bullet Seed (3 hits) vs. 252 HP / 228+ Def Celebi: 99-117 (24.5 - 28.96%) -- possible 5HKO
252+ Atk Life Orb Technician Breloom Mach Punch vs. 252 HP / 228+ Def Celebi: 51-61 (12.62 - 15.09%) -- 9HKO at best
0 SpA Celebi Psychic vs. 4 HP / 0 SpD Breloom: 294-348 (112.21 - 132.82%) -- guaranteed OHKO
Celebi @ Leftovers
Nature: Bold
Ability: Natural Cure
EVs: 252 HP / 232 Def / 20 Spe
• Giga Drain
• Psychic
• Recover
• Thunder Wave/Baton Pass
Any Celebi will wall Breloom to hell and back, but this one especially will demolish him. It doesn't have the best coverage with the given moves, but honestly you can change them to whatever you like, Breloom still isn't getting through anytime soon. Celebi is "immune" to sleep, since switching out will heal you, and is also immune to the rare sub/seed Breloom variant's Leech Seed. A specially defensive set can also be used in order to act as a Keldeo counter as well, because even with special bulk, it would take a Swords Dance and 5 Bullet Seed hits to do serious damage to Celebi, and either way, she can OHKO back with Psychic. Being a counter to two of the tier's top threats is nothing to scoff at, and Celebi is easy to fit onto many teams.
Here are some calcs
252+ Atk Breloom Focus Punch vs. 252 HP / 228+ Def Celebi: 99-117 (24.5 - 28.96%) -- possible 5HKO
252+ Atk Life Orb Technician Breloom Bullet Seed (3 hits) vs. 252 HP / 228+ Def Celebi: 99-117 (24.5 - 28.96%) -- possible 5HKO
252+ Atk Life Orb Technician Breloom Mach Punch vs. 252 HP / 228+ Def Celebi: 51-61 (12.62 - 15.09%) -- 9HKO at best
0 SpA Celebi Psychic vs. 4 HP / 0 SpD Breloom: 294-348 (112.21 - 132.82%) -- guaranteed OHKO