The key thing to remember with this achievement is that everyone has to be out of line of sight when the victim dies. Specimens will only feed if they can't acquire a new target. (N/B: This includes welded doors.) Plus, damaging a specimen causes any nearby specimen with no target, including itself if applicable, to attempt to switch targets to you, so you'll need to either bring a weapon that'll kill them in one hit (explosives are good), or do something extra cheesy like sniping their ankles around a corner (as long as there aren't any others standing around nearby that can still see you).
If you have any qualms about farming achievements, you should probably let them go for this one. Its conditions are such that you'll never accrue any points for it during normal play on a competent team; outside of deliberate setups, said conditions are only ever met when people are either AFK or running around like headless chooks.