Dead Age

The best place to find Undead Scavengers (not to be confused with living scavengers) is in City Day/Night (starting at Area 11), though Undead Scavengers will also show up in Desert Night (again, starting at Area 11), 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 Infected Patients occurs about halfway through the game.

Only enemies labeled as "Undead Scavengers" count for this achievement (yes, it's just one enemy type this time around).

Undead Scavengers
Fast: Yes
Immune to K.O./Stun: No
High Defense (aka needs Breakthrough): Yes
Special Abilities: Virus Bomb (AoE K.O.), Low Blow (Stun)
Priority Targeting Level (1 - 8, 1 is max, 8 is min): 1
Locations: City Day/Night (starting at Area 11), Desert Night (starting at Area 11)