For me personally, the easiest location for this is near the end of chapter 3, Sulaco Falls.
You will come to a long empty corridor, with a window viewing space on your right. You then open a door on your left, and there is a solitary xeno crouched behind the door.
In the right place? Super.
When you open the next door, you'll be in another long corridor, with a window (viewing outer space) on your right. You'll see Xeno's appear, and run away.
This is a pre-scripted event. 4-5 Xenos will exit vents, and run (away from you) towards 4-5 Weyland Mercs. The Xenos attack, and then you have 10 or so people clustered in a very small space. Just throw your firebomb grenade (select it using Y), and wait.
I got this first attempt. Theres just so many enemies in one spot, its hard to miss.