Place the air hammer, using a flat chisel, straight down on the nut (picking the side you want the nut to spin in) and hit one or two small bursts of the trigger. It works absolutely every time!
That said, two things not to do are using a cutting chisel, or keeping your finger on the trigger. FWIW, I've used multiple methods when removing fan clutch nuts and nothing is simpler. As long as you keep the trigger bursts short, there is no harm to the water pump or other parts. The worst that happens is chisel marking on the nut.