It doesn't take a "pvp pro" Just skilled thumbwork and landing plentiful amounts of headshots. An easy strategy, if you're good at timing things, is to prime them for death with a few primary shots, switch to a shotty with good range, blast 'em when just outside their lunge range and mash the thumbstick immediately after. If you timed it right, got lucky, or they were a stupid player, you'll kill them in a most embarrassing and rage-inducing way.