If you want to be mean without punishing survivalist players, you could always go the route of learned resistances.
e.g the more bludgeoning damage you use, the more zombies become resistant over time. There’s also the possibility of applying it powerfully to individual monsters while playing (designed specifically for making the players life hard), spontaneous mutation sort of thing.
In the short term, i’m sure you could just throw up the spawn rate and number of special zombies, but it might get annoying/impossible if not well balanced. If there’s one thing Cataclysm doesn’t have, it’s a difficulty curve. It’s pretty fun till late game though.