Use the Raptor to get this achievement. One headshot will typically down any of the lower level Pacificans and for the ones with armor, it'll take 3-4. Accumulate 25 kills.
Note: Gun kills apparently stack even after death or loading from a checkpoint. If you find a good point in which you are killing many enemies with this gun and you've just saved, press start and select "Load Last Checkpoint" (or whatever it is). You'll reload from the last checkpoint but the game's internal counter doesn't. Thus, the amount of kills you had with this gun prior to your reload will not return to the point of that save.