Juggler is a challenge in Powderkeg Pass. It is unlocked by having three dead Enemigos in the air at the same time.
About halfway through the level, there is a side area with three identical Enemigos who have blue clothes and pointed hats. One is on a ledge and two are on the ground. Find a shotgun and lure the Enemigos close. Stand under the ledge. Shoot up to kill the first Enemigo, then shoot sideways quickly to kill the other two. They will be dead in the air at the same time. Challenge complete!