Great Videos Rich, From what I noticed watching the videos it looks this is how you got to horde. Each shoot to the crosshairs awards a zombie kill, after completing all the crosshairs it arms you with a weapon (axe , gun ,knife ect ect) all the crosshair shoots relight with a yellow light shooting all the yellow light moves you on to the next weapon, rinse and repeat.
It also looks like the "woodbury" shoot back to the plunger lane awards a mystery on the skill shoot (the 2 rollovers and the lanes up top).
Also big points are to be had after going down a lit inlane the next shoot is doubled.