The best level in which to get this achievement is one of the G Corporation levels. Specifically the G Science and Technology Research Building 3. As you are approaching the second corner of the level (where you may notice red lasers ahead of you) you will be forced to fight a string of enemies, ending in a group of Jacks and two large metally opponents.
As soon as the last of these are defeated, 3 aliens will drop down by the red lasers, and you will have a few seconds to kill them before they dissappear. It may be a good idea to let Alise take on the last of the enemies, this way you can position yourself near the lasers ready for the aliens, and also so Alise doesn't kill the aliens, because that wont count.