ALL classes have at least 3 gravestone zombies.
Beastly
-Pied Piper (2 brains)
-Loudmouth (3 brains)
-Squirrel Herder (2 brains)
Brainy
-Zombot Drone Engineer (2 brains)
-Electrician (3 brains)
-Gentleman Zombie (3 brains)
-Kite Flyer (3 brains)
-Mad Chemist (4 brains)
Crazy
-Newspaper Zombie (2 brains)
-Conga Zombie (2 brains)
-Exploding Imp (3 brains)
-Jester (3 brains)
-Abracadaver (3 brains)
Hearty
-Sumo Wrestler (2 brains)
-Landscaper (3 brains)
-Trash Can Zombie (3 brains)
-Rodeo Gargantuar (7 brains)
Sneaky
-Swashbuckler Zombie (2 brains)
-Smelly Zombie (3 brains)
-Stealthy Imp (3 brains)
-Pogo Bouncer (4 brains)
-Firefighter (4 brains)
-Surprise Gargantuar (5 brains)
-Line Dancing Zombie (3 brains)
-Tomb Raiser Zombie (4 brains)
-Mixed-Up Gravedigger (5 brains)
-Any zombie hidden by Mixed-Up Gravedigger or In-Crypted
Grave Buster is pretty good against all Gravestone zombies. You either kill a zombie for the same amount of brains your opponent spent, leaving you even, or you gain a cost advantage if you manage to kill a zombie with a 3 brain cost or more.
You don't need to have 4 copies of Grave Buster in your deck.
With 2 copies you will be just fine, in case you don't find ANY gravestone zombies.
But I don't think it should be changed at all.