Dead Age

The best place to find Undead Prisoners is in Woods Day/Night (especially after Area 11). The Woods, both Day and Night versions, are available right at the start of the game. Because several Undead Prisoner types are only encountered past Area 11, and in a harder zone than the Highway, this may be the fifth or sixth "Kill" achievement you will unlock.

Only enemies labeled as "Infected Prisoner", "Rabid Prisoner" or "Waking Prisoner" count for this achievement.

Infected Prisoner Stats
Fast: No
Immune to K.O./Stun: No
High Defense (aka needs Breakthrough): Yes
Special Abilities: Pool of Bile (Trap), Rancid Breath (AoE Poison DoT)
Priority Targeting Level (1 - 8, 1 is max, 8 is min): 6
Locations: Woods Day/Night

Rabid Prisoner Stats
Fast: No
Immune to K.O./Stun: Yes
High Defense (aka needs Breakthrough): Yes
Special Abilities: Adrenalin Push (Buff Self)
Priority Targeting Level (1 - 8, 1 is max, 8 is min): 6
Locations: Woods Day/Night (starting at Area 11), City Day/Night (starting at Area 11)

Waking Prisoner Stats
Fast: No
Immune to K.O./Stun: Yes
High Defense (aka needs Breakthrough): Yes
Special Abilities: Waking (Stats fight as K.O.ed), Tantrum (Strength increases with each hit sustained), Roundhouse Punch (AoE Attack)
Priority Targeting Level (1 - 8, 1 is max, 8 is min): 1
Locations: Woods Day/Night (starting at Area 11), City Day/Night