Taulima Tautai sealed victory with a four-pointer just three minutes before the end.
The Warriors were first on the scoreboard, thanks to a brilliant individual effort from George Williams.
Joshua Charnley thought he was going to dive across but Shaul had other ideas.
The Black and Whites finished the half with a 12-4 lead after a stunning Sneyd penalty from 55 metres out.
Kirk Yeaman tried to craftily ground the ball out of dummy half from one metre out but was adjudged to have come up short.
Sam Powell helped to bring the Warriors back within two points, after confirmation from the video referee Phil Bentham, powering across out of dummy half.
Fonua denied Dan Sarginson from scoring a certain match-winning try with a brilliant tackle to take the Wigan man into touch.
Minutes later and Tautai secured victory for the Warriors with a brilliant individual effort, reaching over under the posts.