Dead Age

The best place to find Ragers is in City Day/Night (after Area 11), though Ragers will also show up in Woods Day/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 Ragers occurs about halfway through the game. If in a hurry to unlock this achievement, you can farm this achievement in the Woods, but the City tends to be a better explicit farming area.

Only enemies labeled as "Rager" or "Hideous Rager" count for this achievement.

Rager Stats
Fast: Yes
Immune to K.O./Stun: No
High Defense (aka needs Breakthrough): No
Special Abilities: Rage Explosion (K.O. on next turn), Roundhouse Punch (AoE Damage)
Priority Targeting Level (1 - 8, 1 is max, 8 is min): 4
Locations: Woods Day/Night, City Day/Night

Hideous Rager Stats
Fast: Yes
Immune to K.O./Stun: No
High Defense (aka needs Breakthrough): No
Special Abilities: Wild Lunge (AoE), Burning Attack (Fire DoT)
Priority Targeting Level (1 - 8, 1 is max, 8 is min): 4
Locations: City Day/Night (starting at Area 11), Desert Night