They can be split into two groups: Minor bosses and major bosses. Minor bosses have an orange dot on the minimap. They have savage forms like major bosses, which have heavily increased stats. However, minor bosses do not have corresponding elites, meaning that minor bosses cannot have zombie modifiers. They are treated like regular zombies in a way that they spawn in large numbers and swarm the players. Minor bosses do not guarantee a strongbox when killed.
Most major bosses are stronger than minor bosses, except they spawn only at the end of each mission except VIP and maps on Nightmare Mode, either alone, in groups of two or more, or as a combination of different bosses. Major bosses have a larger orange dot on the minimap than the minor ones. The major bosses can be an elite which can have one of 4 modifiers, Extra Strong, Berserk, Extra Fast, or Dark Minion. Boss elites appear at later levels compared to bosses that are not elites, or even two copies of the boss. Major bosses have a 100% chance to drop a strongbox, except when facing multiple major bosses, only one of them will drop a Strongbox. Because no major bosses spawn in VIP, VIP is the only mission that does not guarantee at least one Strongbox at the end of a mission.
Savage bosses will appear in high levels, having exponentially increased health in addition to high damage (often ten times that of the regular boss) and a more hellish appearance. Their increased health isn't necessarily a problem on its own, but certain bosses can dish out major damage in their savage form with this trait, a fact most notable with the Savage Necrosis and on Vaccine.
- Zombdroid Servant - Only appears in the missions VIP and Power Out
- Zombdroid Soldier - Only appears in the missions VIP and Power Out