Dead Age

The best place to find Undead Scientists is on Desert Night. Because Desert Night only unlocks around Day 35, the Undead Scientist set of achievements will be one of the last "Kill" sets of achievement you will unlock, as the game ends on Day 42. You may have to farm the Desert Night a bit toward the end of the game to complete this set of kill achievements, since the opportunity to unlock them occurs late in the game (though you probably only need to make one or two extra trips to Desert Night to do so).

Only zombies labeled as "Undead Lab Assistant" or "Undead Scientist" count for this achievement.

Undead Lab Assistant
Fast: No
Immune to K.O./Stun: No
High Defense (aka needs Breakthrough): No
Special Abilities: Rancid Breath (Poison AoE), Body Armor (Buff)
Priority Targeting Level (1 - 8, 1 is max, 8 is min): 4
Locations: Desert Night

Undead Scientist
Fast: No
Immune to K.O./Stun: No
High Defense (aka needs Breakthrough): No
Special Abilities: Spew Poison (Poision DoT), Regenerate (Heal Self)
Priority Targeting Level (1 - 8, 1 is max, 8 is min): 7
Locations: Desert Night