That skill is barrage. Barrage looks more like a dance than a skill. I use Barrage to Dragon Strike for single form bosses (and most training). Sometimes I throw in a Cork Screw to KB or give more invincibility.
Headless horseman (as rasu said) or Papulatus. Pianus is a ***** to kill, although it has decent skillbook drops. If you count the first ToT boss, or Mannon/griffey, then those are the 2nd easiest.