Dead Age

The best place to find Undead Soldiers is in Woods Day/Night (after Area 11), though Undead Soldiers will also show up in Desert Night (after Area 11). As well, undead soldiers appear with the Highway Day/Night Boss fight in Area 20. 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 "Infected Soldier" or "Undead Grenadier" count for this achievement.

Infected Soldier Stats
Fast: No
Immune to K.O./Stun: Yes
High Defense (aka needs Breakthrough): Yes
Special Abilities: Awareness (Counters Melee Attacks)
Priority Targeting Level (1 - 8, 1 is max, 8 is min): 6
Locations: Woods Day/Night (starting at Area 11), Desert Night (starting at Area 11), Highway Area 20 Boss Fight

Undead Grenadier Stats
Fast: No
Immune to K.O./Stun: Yes
High Defense (aka needs Breakthrough): Yes
Special Abilities: Shrapnel Grenade (AoE)
Priority Targeting Level (1 - 8, 1 is max, 8 is min): 6
Locations: Woods Day/Night (starting at Area 11), Desert Night (starting at Area 11), Highway Area 20 Boss Fight