A double-strength shot fired out of a hole in the floor at a province boss, then point-blank into a mantaur's face.
Donut announces in advance that it is going to be a good one: she has a full-powered shot stored in her Prism Goggles and intends to hit the Province Boss with a double shot. She leans into the raised The Def Leppard Cart, fires, scrambles back and announces the hit twice — and the boss bellows with a thousand mouths and charges into the portal. Carl predicts, correctly, that she will collect an achievement for the shot as well as for looking at the thing.
In the tunnel ambush she uses it far less ceremoniously: one missile into the second ManTauR as the smoke drops, then point-blank missiles pumped into its head from Mongo's back, and a final one into its face while Carl is screaming at her to stop before she kills it.
Donut's workhorse attack spell, and the shot that baits a province boss into a portal.
Also appears in books you've already read — spoiler-safe to their endings: