The home side sat back for the remainder of the half and Bromley hit back just before the break, Sean Devine heading home via the crossbar.
The Ducks stepped up a gear in the second period though. Former Wycombe Wanderers midfielder Andy Robinson restored United's two goal advantage in the sixty sixth minute, and Steve Grenfell reacted quickest, after a Darren Collins effort had been saved, to make it four three minutes later.
United weren't finished yet as Cliff Hercules helped himself to his second nine minutes from time, and a Phil Heath run and cross was converted by Darren Seabrook to complete the rout.