Dead Age

The best place to find Undead Cops is in Highway Day/Night (after Area 11), though Undead Cops will also show up in Woods Day/Night (after Area 11) and the City (after Area 11). Since "after Area 11" means a pretty significant jump in difficulty, farming these enemies is more of a mid-game activity rather than something that can be done out of the gate.

Only enemies labeled as "Bitten Police Officer" or "Infected Police Officer" count for this achievement.

Bitten Police Officer Stats
Fast: No
Immune to K.O./Stun: No
High Defense (aka needs Breakthrough): Yes
Special Abilities: Blind (Debuff), Poison (AoE Poison DoT)
Priority Targeting Level (1 - 8, 1 is max, 8 is min): 7
Locations: Highway Day/Night (starting at Area 11), Woods Day/Night (starting at Area 11), City Day/Night (starting at Area 11)

Infected Police Officer Stats
Fast: No
Immune to K.O./Stun: Yes
High Defense (aka needs Breakthrough): Yes
Special Abilities: Awareness (Counters Melee Attacks), Take Aim (Buff)
Priority Targeting Level (1 - 8, 1 is max, 8 is min): 6
Locations: Highway Day/Night (starting at Area 11), City Day/Night (starting at Area 11)