Dead Age

The best place to find Undead Civilians is in City Day/Night, though Undead Civilians will also show up in Desert Night (abet infrequently). Since City Day/Night, isn't available right at the start of the game (it opens up on Day 22), your best farming opportunity for Undead Civilians occurs about halfway through the game.

Only enemies labeled as "Infected Civilian" or "Undead Civilian" count for this achievement.

Infected Civilian Stats
Fast: No
Immune to K.O./Stun: No
High Defense (aka needs Breakthrough): No
Special Abilities: Shriek (Summons Bitten Pedestrian when hit)
Priority Targeting Level (1 - 8, 1 is max, 8 is min): 2
Locations: City Day/Night, Desert Night

Undead Civilian Stats
Fast: No
Immune to K.O./Stun: No
High Defense (aka needs Breakthrough): No
Special Abilities: Awareness (Counters Melee Attacks), Rancid Breath (AoE Poison DoT)
Priority Targeting Level (1 - 8, 1 is max, 8 is min): 7
Locations: City Day/Night, Desert Night