Phantoms are the large blueish aliens that teleport around the area. Sometimes when you shoot they teleport out of the way. To get the achievement you (and not another player) have to kill 5 phantoms. The easiest way is to play with friends and take turns getting the achievement. There is 1-2 in the first area (before the first encounter with the breeder) 1-2 in the compound after the breeder, and 2-3 inside the facility, so as long as you get to the end you should aquire this achievement.