when building the shapes during a Composition Spell, you will swipe to activate each "side" (technically you will light up the EDGES) of the shape. they will turn from plain gray to rainbow colored to show they are active. keep an eye on which ones you have activated. for this achievement, you must activate all the "sides" (edges) EXCEPT ONE. just leave one gray and make sure not to swipe when the cue comes up for that one.
EDIT: thanks to Xpovos for pointing out that the game says "side" when it means "edge". see their comment below for more info if the distinction is unclear.