The Mariners were never threatened by the Reds as they cruised to a 3-0 lead at half-time courtesy of goals from Mustafa Amini, Bernie Ibini and a Patrick Zwaanswijk penalty before Rostyn Griffiths completed the rout in the second-half.
Amini, on loan from Borussia Dortmund, opened the scoring after 29 minutes when, after hovering in a pocket of space behind Adelaide's defence, he made an intelligent run that Pedj Bojic recognised with a neat through ball, allowing the 18-year-old to finish past Eugene Galekovic with ease.
Adelaide barely had time to regather themselves when Ibini made it 2-0 just three minutes later when he held back his run into the six-yard area, catching the Reds' defence off-guard, before smashing Michael McGlinchey's smart cut-back into the roof of the net.
It got worse for the Reds five minutes later.
United captain Jon McKain attempted to head the ball back to Galekovic when Ibini nipped in and wrestled the ball off the Reds defender only for his shot to strike the underside of the bar.
Play was called back however by referee Kris Griffiths who awarded a penalty to the Mariners after McKain was deemed to have tugged back Ibini, allowing Dutchman Zwaanswjik to step up and dispatch the penalty comfortably.
The Mariners added a fourth when Griffiths nodded home a free header from McGlinchey's long free-kick in the 66th minute.