Call of Duty: Black Ops III Acted Alone Achievement Guide
Best way I found was to do this in CO-OP with some friends. You'll want to turn the Power on from the Jugger-Nog side of the map (left side of spawn) I would also recommend buying Jugger-Nog (2500 points).
A useful GobbleGum I would recommend using is Immolation Liquidation
(Fire Sale) to get a good weapon from the box.
When you have Jugger-Nog and a good weapon, go to where the KRM-262 is located (Double-Barreled Shotgun in WaW) (up the stairs from the left spawn) and overlook the courtyard. A headshot give you 100 points (200 if double points is active).
Your friends will then want to cover the stairs and the wall directly behind you, whilst you shoot the zombies in the courtyard.
Hope this helped.