Managed to get this when you first fight against him. Pretty easy if you know what to do.
First, kill all the zombies, use your grenades to kill groups of them, while avoiding the chainsaw dude.
Once all the zombies are out of the picture, use a machine gun to whittle the Chainsaw Elites health a bit (about 100 rounds should suffice) and a few mines if you have them (if you dont have the mines, use 200 machine gun rounds).
Now, stay a short to moderate distance and he will raise his chainsaw up for a few seconds and charge you, followed up by a few swings. THIS IS YOUR OPENING.
He will stagger after the swings, allowing you to get close and shoot him (make sure you're in ADS!!) point blank.
Once he starts his death animation, stand on top of him and hold
, once he becomes lootable, the "loot circle" will automatically begin to fill.