Dwimmer

"It is ill dealing with such a foe: he is a wizard both cunning and dwimmer-crafty, having many disguises."

—Éomer's comment about Saruman

Created word. The verb dwimmer might be a variant from the old English word "dwimor" which means illusion.

"dwimmer-crafty" could translate as Knowledgeable, crafty, or skilled in the art of magic.