BioShock Infinite: The Complete Edition

Immediately after you get the Peeping Tom Plasmid in the Cupid room early on in the Episode, your game will autosave. Two splicers will appear in the adjacent room. All you need to do is hold LT to become invisible and knock out both splicers with Y. Since your game autosaved, rinse and repeat by reloading the checkpoint. You can most likely get many kills throughout regular gameplay, but this solution is for those gamers who just want to get the achievement as quickly as possible.

Another good point to get this is when you get to the big room to get the Bathosephere part. There are plenty of enemies and there is a good checkpoint to repeat over and over again!