Image: Naughty Dog / PlayStationOfficial screenshot from Naughty Dog / PlayStation.
Skillz is the bronze trophy for winning the water gun fight against Riley in Left Behind. The guns never run dry but need constant pumping, and each girl has a soak meter: fill Riley's before she fills yours.
Keep pumping between bursts so the gun never sputters. After the water fight, a best-of-three cat and mouse round follows where a single hit wins each round.