breloom is pretty tricky. its somewhat rare, but if your team lacks a breloom counter he could easily sweep your entire team.
other than overspecialized counters for breloom, such as hypno or ariados, there are a few common counters for breloom that can also accomlpish other feats:
sleep talk cresselia
sleep talk heracross
after a pokemon has been put to sleep:
bulky gyarados with ice fang or return
salamence in general
gengar
crobat
etc.
just put one of the above on your team and keep him alive and breloom will not be too much of a problem
other than overspecialized counters for breloom, such as hypno or ariados, there are a few common counters for breloom that can also accomlpish other feats:
sleep talk cresselia
sleep talk heracross
after a pokemon has been put to sleep:
bulky gyarados with ice fang or return
salamence in general
gengar
crobat
etc.
just put one of the above on your team and keep him alive and breloom will not be too much of a problem