Once you have left the mines with Alex and the vortiguant, you will be attacked by regular and acid antlions around a bunch of repellant towers. After a bit of fighting and moving through the area, Two antlion guards (The biggest antlions that only crop up now and again). Just kill them. Now this might be a bit harder because you don't have the pulse rifle yet. Your best bet is the shotgun. Just keep pumping them, focus on one then the other. The achievement unlocks after you kill them both.