Chapter 2 "Evacuation Orders" -- Rooftop Showdown, after a cutscene you'll go down a stairs and manually open a door (Rico mentions "I got you covered") and appear behind a lone Helghast. To his right beyond some debris there are 4-5 enemies shooting towards the streets, cook a grenade while going over the debris and throw it at them. You need to be fast as they will spot you instantaneously when you go over the debris. If you fail, you can restart from last checkpoint, then cook the grenade immediately after you open the door and throw it at the target location before dealing with the lone Helghast in front of you. Tip: You can observe how long you've cooked the grenade by looking at the bars on the grenade when you're holding it.
The above excerpt was extracted from "Killzone 3 Trophy Guide" at PS3Trophies.Org.