Cough Syrup works extremely well with Rainbow Barfer, allowing it to deal up to 34 damage on an unguarded lane. When combined with Finn or BMO's abilities or Poultrygeist's ability, one can summon Rainbow Barfer and use Cough Syrup on the same turn, allowing incredible damage (or, more likely, a win) before the opponent has time to counter.
Having similar landscapes hinders Rainbow Barfer's Floop ability. There is no solid counter to it because it is a very versatile creature and is not weak to Cerebral Bloodstorms. High damaging creatures that can deal lots of damage is an option, but tank creatures that can survive its floop ability also work.