Basically you take a ranger who has the AA Headshot as high as he can get it, Trick Arrow as high as he can get it, and the spells Hail of Arrows/Arc of Arrows/Barrage of Arrows(Hail of Arrows preferred for mana reasons) and Vinelash Cascade, then you goto a zone that has headshottable critters, generally humanoids and other things with heads, then you round up a fairly large train of them, pack them into a tight group, and unleash Vinelash on them. This is a root with a snare trigger effect. Then you spam Hail at your rooted mobs and watch the headshots pop them like blisters. Keep autofire going during this time and target mobs as you go, Trick Arrow gives you two headshot chances if it fires.
All that's required, a ranger with a decent mana pool, a bow, the spells and the necessary AAs.