After scouting, I knew where my opponent would be, so I took some time to flank around to a better position for myself. I then knew that if I just shot while he was near cover, chances are I wouldn't be able to kill him. I threw a grenade towards the cover he would definitely go for when I would shoot him while he's exposed, so he'd get his face blown off before he would get there.
If they are running from you, do not head straight towards them. Guess where they will go, and cut them off. Greet them with a nice grenade or OSMAW to the face.