There are only three pneumatic weapons within the game that will count for this achievement: Tikhar, Helsing, and Hellbreath. Obtaining fifty kills on human enemies does not have to be consecutive nor with the same weapon, so any kills with pneumatic weapons will add up over the time of your playthrough(s).

A perfect opportunity to begin working on this achievement is on the Frontline level, providing you purchased one of the pneumatic weapons at the Armory level just before. With that said, both "Frontline" and "Black Station" house plenty of human enemies so you might be able to get this done within those two levels alone. Either way, once you have killed at least fifty human enemies with the pneumatic weapons, the achievement will be yours.