I have an idea for a boss / tough normal zombie, maybe called the "Hive" zombie or something
It periodically spawns weak, smaller zombies in groups of five or ten as well as doing somewhat weak attacks
Would be different from most of your tough zombies, which all seem to just be high health and high damage.
Each of the spawned zombies should be pretty weak individually but with waves spawning fairly quickly they could overwhelm players that weren't cautious.