Perfect Farm Spot.
Load Mission 17 get out of the Elevator Equip your Shotgun and Sprint to the Armed Survivor to your left and shoot him (will also net you the kill 10 Humans Achievement) and the Alien will Jump out of the Ventilation Shaft and Kill you.
Load your most Recent Save.
Rinse and Repeat.
P.S: Make sure you play on Hard Difficulty so the Alien will find and kill you alot faster also keep firing your Gun after shooting the Guy.