After you free Keyes then most of the Xeno seem to come from vents scattered throughout the areas. Specially when you get to the part where you and O'Neal are going through the catwalks. You don't have to kill them as they come from the vent though. I got it fairly easily, I believe either as long as they came from a vent or are just outside the one they came from the kill will count.
If it proves difficult then you can always use a grenade launcher and hit them as they appear, then die and do it again from the checkpoint. Unless it says 'in one campaign mission'. I believe those challenges reset if you die.
GT: XeroStrife PSN ID: Xero_Strife Currently Playing Most: Guild Wars 2, Castlevania Harmony of Despair, Aliens: Colonial Marines and Resident Evil 6