You will more than likely earn this achievement just naturally playing through the game. I unlocked this in the Undercity.
All kills count for any weapon that you use. The achievement is unlocked upon the 100th kill.
Mutants include Watchmen, Spiders, Shrimps and Demons. Small, non-hostile mutants, such as Mutant Crows and Shrimp Hatchlings do not count. DLC mutants also do not count.