Check: Mega Venu
Counter: Buzz
Feel like Volcarona is vulnerable to the rare SD + Stone Edge offensive variant and the increasingly common CB Stone Edge whereas Mega Scizor provokes more telegraphed sequences that can be troublesome if Bulu is paired with Magnezone or can lead to trouble if it is the rare Z-Fight, especially if paired with Spikes. All things considered, I reckon these two are the best direct answers in each category, but you cannot really go wrong regardless of which you pick as they all do the trick far more often than not.
EDIT: did NOT see it was only CB Bulu, my bad for not reading fully. Either way, these two are still great examples of checks and I still am not too fond of Volcarona being picked then. Mega Scizor is another great counter then though, yea.